Publishing type：Research paper (scientific journal)  

Biosensors in the Terahertz (THz) region are attracting significant attention in the biomedical and chemical analysis fields owing to their potential for ultra-trace sensing of various solutions with high sensitivity. However, the development of compact, highly sensitive chips and methods for easy, rapid, and trace-amount measurements have been significantly hindered by the limited spatial resolution of THz waves and their strong absorption by water. In this study, we developed a nonlinear optical crystal (NLOC)-based compact THz sensor chip, and a near-field point THz source with a diameter of ~ϕ20 μm was locally generated via optical rectification. Here, only the single central meta-atom was excited. The reflective resonance responses highly depend on the array number and period of the meta-atom structures. The sensing performance was examined with several liquid biological samples, such as mineral water, DNA, and human blood. 1 μL of samples was directly dropped onto the meta-surface with an effective sensing area of 0.32 mm2 (564 μm × 564 μm). Obvious resonance frequency shifts were clearly observed. This research holds significance in advancing liquid bio-sample sensing methodologies by facilitating easy, rapid, and trace-amount measurements and promoting the development of compact and highly sensitive THz sensors tailored for liquid biological samples.

DOI： 10.3390/photonics11080766

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DOI： 10.3390/ma17071497

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Language：English   Publishing type：Research paper (scientific journal)  

<jats:p>We present a novel technique for capillary electrophoresis (CE) using terahertz (THz) waves, namely “THz-CE,” which enables us to sensitively detect separated substances in a solution flowing in a hollow of capillary whose inner diameter is smaller than 100 µm. Such THz detection could be achieved by utilizing the near-field interaction between a solution filled in a capillary and a point THz source that was locally generated by optical rectification in a nonlinear optical crystal irradiated with a femtosecond pulse laser. Here, we investigated the performance of THz-CE numerically and experimentally, and succeeded in observing the electrophoretic chromatogram for the separation between acetic acid and <jats:italic>n</jats:italic>-propionic acid by THz-CE.</jats:p>

DOI： 10.1364/ome.500594

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DOI： 10.1038/s43246-023-00428-6

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Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

DOI： 10.1117/12.2681961

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.chroma.2023.464384

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.nanolett.3c00765

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Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

<jats:title>Abstract</jats:title>
<jats:p>Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHz–∼30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon <jats:italic>et al</jats:italic> 2017 <jats:italic>J. Phys. D: Appl. Phys.</jats:italic>
<jats:bold>50</jats:bold> 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a ‘snapshot’ introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation.</jats:p>

DOI： 10.1088/1361-6463/acbe4c

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DOI： 10.1117/12.2657052

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Bismuth telluride (Bi2Te3)-based heterostructures have attracted considerable attention owing to their interesting anisotropic properties and expected higher thermoelectric performance. Therefore, exploring the nature of the carrier dynamics in these heterostructures has been an important subject in the design and optimization of advanced materials. In the present study, hot carrier injection and its subsequent spatiotemporal behavior in a multilayered crystalline Bi2Te3-Tellurium (Te) eutectic composite were studied using a laser terahertz (THz) emission microscopy (LTEM). The THz emission electric fields at the Bi2Te3-Te interface were polarized perpendicular to the interface. The polarities of these waveforms reveal the direction of the electric field between the Bi2Te3 and Te regions, indicating the carrier types of these components and the p-n junction formed at the interface. In addition, in the Te region, a strong THz emission with an electric field polarized parallel to the interface was observed. This unique THz emission can be qualitatively explained through hot photocarrier anisotropic transport by considering the effective mass of electrons and holes. LTEM clarified the local carrier dynamics in the microstructures and revealed the potential distribution and anisotropic transport properties. These findings contribute to the exploration of eutectic heterostructures as new functional materials and provide new avenues for cutting-edge thermoelectric and photovoltaic devices.

DOI： 10.1063/5.0137862

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This study conducted laser-induced terahertz emission spectroscopy on a VO₂/Si heterojunction. Consequently, rapid estimation of the local interface potential was realized and the work function of VO₂ was obtained as 5.17–5.25 eV with increasing temperature from 320 to 380 K. Moreover, an obvious terahertz emission variation was observed across the metal–insulator phase transition of VO₂, and the doping conditions of the Si substrate largely influenced the terahertz emission. These results imply a strong relationship between the terahertz emission amplitude and the interface electric field, which supports the rapid performance of terahertz emission spectroscopy in estimating the work function of VO₂.

DOI： 10.1063/5.0136610

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The transmission spectra of terahertz (THz) waves through Fe doped β-Ga2O3 in the range of 1-6 THz by air-plasma-based THz time-domain spectroscopy (THz-TDS) at room temperature and their temperature dependence between 90 and 500 K in the range of 0.7 THz-1.8 THz have been measured. We estimated the complex dielectric constants by fitting the spectra with the classical damped oscillator dispersion model and obtained the anisotropic lowest transverse optical (TO1) phonon frequencies. The values at room temperature agreed with the reported values, and no significant temperature dependence was observed.

DOI： 10.35848/1347-4065/acae52

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Language：English   Publishing type：Research paper (scientific journal)  

This study utilized a multiple-input multiple-output synthetic aperture radar (MIMO-SAR) technology employing a frequency-modulated continuous wave (FMCW) millimeter-wave MIMO radar to execute two-dimensional (2D) and three-dimensional (3D) imaging of objects. By analyzing the intermediate frequency (IF) signal generated from the transmitted and received chirp signals, Range FFT spectra with information on the distance to the object and on the reflection intensity according to frequency were obtained. The integration of reflection intensity provided a comprehensive image at varying distances, while the 2D cross-sectional image of an object inside a concealed cardboard achieved a spatial resolution of less than 1 mm. Through innovative data processing techniques, such as the spatial translation filter, we succeeded in the reconstruction of a monolayered 3D image of a clothed human body's surface. Moreover, the system demonstrated its potential for security checks by effectively identifying dangerous items concealed by human.

DOI： 10.1109/ACCESS.2023.3302889

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Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Lenses/mirrors with fast data acquisition speeds and extended focal depths have practical importance in terahertz (THz) imaging systems. Thus, a high-speed 600 GHz-band THz imaging scanner system with enhanced focal depth is presented in this work. A polygon mirror with a 250 Hz scanning frequency and an integrated off-axis parabolic (OAP) mirror with an ~170 mm focal depth were employed for 2D imaging. The simulation and experimental results show that a spatial resolution of ~2 mm can be achieved as the imaging distance varies from ~85 to 255 mm. The proposed system was applied to image a hidden metal object as a potential security application, demonstrating that this system can image targets with an enhanced focal depth.

DOI： 10.3390/photonics9120913

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DOI： 10.1038/s41377-022-01033-x

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Terahertz (THz) emission from the silicon (Si) surface by the femtosecond laser illumination shows a lot of information, known as the THz emission spectroscopy (TES) or the laser-induced THz emission microscope (LTEM), which seems to be the promising method for evaluating the surface/interface properties of Si-based devices, such as Si metal-oxide-semiconductor (Si MOS) structure. To enlarge the application and explore silicon electronics, it is necessary to build a theoretical model, which can accurately and simply describe the relationship between the THz emission amplitude and the external bias voltage on the Si MOS structure. Here, we focus on the THz emission spectrum from the p-type Si MOS structure and discuss the THz emission field under different DC bias conditions. The theoretical model is derived from Poisson's equation and shows great consistency with the experiment results. Besides that, the flat-band voltage, interface trap states, and photo-Dember effect have been discussed to modify the model.

DOI： 10.1117/12.2632893

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Anisotropic conductivity and its temperature dependence of unintentionally doped and Fe-doped bulk (010) beta-Ga2O3 were studied by using terahertz time-domain spectroscopy (THz-TDS) with the THz polarization being parallel to the [100] and the [001] directions. The responses to the THz waves were different along the a-axis and the c-axis, where the Drude and the Drude-Smith model were applied for fitting the data of these two axes, respectively. This suggests that the carriers travel along the a-axis as free carriers while those are highly localized along the c-axis. The unintentionally doped sample showed a unique temperature dependence, in particular, a decreasing trend of the real part of the complex conductivity with increasing temperature. This is attributed to carriers being completely thermally excited from shallow impurity levels, and afterward, the scattering behavior has been enhanced, causing the conductivity to decrease. Conversely, the Fe-doped sample showed little temperature dependence, indicating this sample to be thermally stable. Moreover, the mobility along the a-axis in the Fe-doped sample was estimated to be much higher than that along the c-axis.

DOI： 10.1007/s10762-022-00868-0

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We developed a nonlinear optical crystal-based compact terahertz (THz)-microfluidic chip with several I-design meta-atoms for attomole (amol)-level sensing of trace amounts of solution samples. The I-design meta-atom consists of a metallic strip with a micometer-sized gap sandwiched by other metallic strips. It is periodically arrayed in a row of 1 × 5 units. A point THz source locally generated by optical rectification at the irradiation spot of a femtosecond-pulse laser beam induces a tightly confined electric-field mode at the gap regions and modifies the resonance frequency when a microchannel fabricated along the space between the metallic strips is filled with solutions. Using this chip, we could detect minute changes in the concentration of trace amounts of ethanol- and glucose-water solutions and mineral water by measuring the shift in the resonance frequencies. We succeeded in detecting 472 amols of solutes in solutions less than 100 picoliters using an effective sensing area of 0.10 λ_THz × 0.57 λ_THz, which indicates an improvement of approximately one order of magnitude compared to the performance of existing chips. This technique compactifies THz-microfluidic chips with high sensitivity and would accelerate the development of microfluidics integrated with THz technology, such as lab-on-a-chip devices and THz micro total analysis systems.

DOI： 10.1088/2515-7647/ac691d

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Other Link： https://iopscience.iop.org/article/10.1088/2515-7647/ac691d/pdf

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We report the terahertz emission spectroscopy (TES) and photoluminescence (PL) spectroscopy results for a semi-insulating (SI) GaN film in comparison with those for unintentionally doped (UID) and magnesium (Mg)-doped ones. The TES and PL results showed notable slow changes on a time scale of approximately 10 s for the SI and UID GaN films, but not for the Mg-doped GaN film upon femtosecond ultraviolet laser illumination. The origin of the slow responses of the TES and PL spectra was studied by observing them under ambient air and vacuum conditions and attributed to hydrogen dissociation from GaN as a result of optical excitation. Published under an exclusive license by AIP Publishing.

DOI： 10.1063/5.0086788

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{MDPI} {AG}  

The instantaneous photocarrier transport of perovskite solar cells was evaluated by assessing laser-induced terahertz (THz) emission to understand carrier dynamics in perovskite solar cells. The waveform of laser-induced THz radiation from an interface between the TiO2 electron transport layer and perovskite active layer of an n-i-p perovskite solar cell with varying external bias was measured using THz-time domain spectroscopy. The amplitude of the THz radiation decreased with increasing reverse bias voltage. The waveform of the THz radiation was inverted at a strong reverse bias. The measured bias voltage dependence suggests that the transient current generated at the interface between perovskite and TiO2 owing to the higher mobility of electrons than that of holes, namely the photo-Dember effect, is the dominant source of THz radiation and the destructive contribution of the interfacial electric field inverts the transient current when a reverse bias causes a strong interfacial electric field. The significant contribution of the interfacial electric field has not been previously reported in perovskite thin films and is unique to solar cells. We believe that band bending at interfaces in perovskite solar cells will be determined from the THz emission with proper modeling.

DOI： 10.3390/photonics9050316

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<jats:p>Beta gallium oxide, β-Ga2O3, is one of the promising ultrawide bandgap semiconductors with a monoclinic (C2/m) β-phase structure showing strong anisotropic properties. To improve the performance of these devices, more optical characterization is required. Here, the anisotropic carrier dynamics in optically excited (010) oriented Fe-doped β-Ga2O3 was studied by terahertz time-domain spectroscopy. An 800 nm continuous-wave light source was employed to excite carriers from Fe acceptors to the conduction band and to probe an anisotropic interaction with the THz field. The complex THz conductivities fitted with the Drude and Drude–Smith models revealed that the optically excited carriers behave as nearly free carriers along the a-axis, whereas those along the c-axis show a non-Drude type response. The estimated mobility for the c-axis agrees with the reported values, whereas the results suggest much higher mobility along the a-axis.</jats:p>

DOI： 10.3390/photonics9040233

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

Terahertz imaging is an emerging candidate to diagnose breast cancers in a labelfree manner. However, detailed terahertz analysis of early stage breast cancers is difficult to achieve owing to its low spatial resolution. In this study, utilizing a probe-less terahertz near-field microscope named scanning point terahertz source microscope, we visualize an unstained comedo ductal-carcinoma-in-situ including an architectural structure (comedo necrosis) measuring similar to phi 500 mu m, which is known as highly-malignant early-stage breast cancer, in terahertz images for the first time. The outcome is a critical step toward the label-free diagnosis of single early stage cancer lesions with terahertz waves. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

DOI： 10.1364/OPTCON.448444

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We studied the phased-array effects in terahertz emission from semiconductor surfaces upon femtosecond laser illumination. A finite-difference time-domain simulation and experimental observation of the radiation patterns were utilized to examine the optical excitation at normal to the semiconductor surface and oblique angles of 30 & DEG; and 45 & DEG; with diameters of & SIM;10 and 275 mu m. The results revealed that there exists a clear phased-array effect for the defocusing conditions. The larger diameter induced a pronounced directivity of the emission owing to the constructive interference of multiple point sources obeying the law of linear superposition, whereas the radiation patterns at smaller diameters were explained as the dipole point source. This finding, in addition to previous studies, will provide a better understanding and contribute to applications of terahertz emission spectroscopy in the field of semiconductor research and development.& nbsp;(C) 2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

DOI： 10.1063/5.0077054

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

Terahertz (THz) emission spectroscopy (TES) was used to evaluate the properties of interfaces between thermally grown oxides and 4H-SiC(0001) substrates. Metal-oxide-semiconductor (MOS) structures with transparent electrodes were irradiated with a femtosecond laser pulse and the emitted THz signal was measured by changing the applied gate voltage. The amplitude of the THz pulse signal is dependent on the electric field, namely, band bending near the SiO2/SiC interfaces, and thus contains information on the change in the surface potential of the SiC MOS structures. We compared the peak THz amplitude (E-THz) and gate voltage (V-g) curves taken from SiC MOS structures with different interface qualities and observed a steep E-THz-V-g curve for a high-quality SiO2/SiC interface as compared with the curve for a structure with a higher interface state density. We also compared the E-THz-V-g and capacitance-voltage characteristics of SiC MOS capacitors and investigated the mechanism of THz emission from the SiC MOS structures to validate the ability of the TES technique for characterizing SiO2/SiC interfaces.& nbsp;Published under an exclusive license by AIP Publishing.</p>

DOI： 10.1063/5.0058962

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Metal-insulator-semiconductor (MIS) is an essential structure in semiconductor devices. Owing to the increasingly complex development of semiconductor devices, the local information of MIS, such as passivation, defects, and transient carrier dynamics, is becoming more challenging to characterize. Laser-excited terahertz (THz) emission spectroscopy and imaging are promising for investigating such local properties in a noncontact and nondestructive manner; however, the discussions are speculative and phenomenological. In this study, we formulate the mechanism of THz radiation based on a Si metal-oxide-semiconductor (MOS) structure to semi-quantitatively evaluate the MOS devices. A simplified model for both n- and p-type Si MOS structures is derived from Poisson's equation, which can effectively explain the dependence of THz emission amplitudes on external bias voltages considering external effects, including the flatband voltage, hot carrier diffusion, and quick recombination through interface states in addition to the photocurrent due to photocarrier field acceleration. The result reveals that one can estimate various "local" parameters such as the surface potential, semiconductor doping level, and dielectric constant of the passivation layer. The simplified model fills the gap between theory and observation, resulting in an advanced analytical tool for semiconductor research and development.

DOI： 10.1063/5.0055671

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This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices. Published under an exclusive license by AIP Publishing.

DOI： 10.1063/5.0047402

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As a promising terahertz (THz) source, two-color laser filamentation in gases attracts an exponential growth of interest because of the scalability of output THz pulse energy and bandwidth with input parameters. However, nonlinear effects within the laser filament on the THz pulse generation process, such as intensity-dependent refractive index change induced additional phase, have never been studied in depth. In this paper, we experimentally investigate the THz waveform evolution with input infrared pump energy from 1.75 similar to 2.75 mJ were recorded by an air biased coherent detection (ABCD) sampling setup. We observe that the magnitude ratio of the positive and negative peaks increases with the pump pulse energy, and the smooth peak at the tail gradually disappears. Based on the photocurrent model, our calculation shows that the modulation is caused by the plasma-induced additional phase that varies with incident laser intensity. Additionally, the smooth peak at the tail is considered to be caused by the fading of the residual current after the negative peak.

DOI： 10.1007/s10762-021-00797-4

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Terahertz (THz) emission spectroscopy and microscopy are applied to investigate the electron and lattice dynamics of Ga0.8In0.2N/GaN multiple quantum wells (MQWs). The THz emission consists of three distinct, differently timed signals, whose physical mechanisms are attributed to i) laser-induced ultrafast dynamical screening of built-in bias electric field in MQWs followed by ii) capacitive charge oscillation of the excited carriers and iii) the coherent acoustic phonon (CAP)-driven polarization surge at the discontinuity between the GaN capping layer and air. These multifunctional optical responses show strong dependence on the quantum well width and photon energies. The temporal separation between the first and third THz pulses corresponds to the propagation of the CAP across the GaN capping layer of the MQW structure, whose thickness can thus be determined with 10 nm precision.

DOI： 10.1002/adom.202100258

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.202100258

Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Terahertz-light imaging is attracting great attention as a new approach in non-invasive/non-staining biopsy of cancerous tissues. Positively, terahertz light has been shown to be sensitive to the cell density, the hydration content, and the chemical composition of biological samples. However, the spatial resolution of terahertz imaging is typically limited to several millimeters, making it difficult to apply the technology to image biological tissues which have sub-terahertz-wavelength-scale inhomogeneity. For overcoming the resolution, we have recently developed a terahertz near-field microscope with a spatial resolution of 10 µm, named scanning point terahertz source (SPoTS) microscope. In contrast to conventional far-field terahertz techniques, this microscope features the near-field interactions between samples and point terahertz sources on a sub-terahertz-wavelength scale. Herein, to evaluate the usefulness of terahertz imaging in cancer tissue biopsy in greater detail, we performed terahertz near-field imaging of a paraffin-embedded human-breast-cancer section having sub-terahertz-wavelength-scale inhomogeneity of the cancer cell density using the SPoTS microscope. The observed terahertz images successfully visualized local (~250 µm) inhomogeneities of the cell density in breast invasive ductal carcinoma. These results may bypass the terahertz limitation in terms of spatial resolution and may further motivate the application of terahertz light to cancer tissue biopsy.

DOI： 10.3390/photonics8050151

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

As a state-of-the-art electro-optical crystal, zinc telluride (ZnTe) is widely used in terahertz (THz) emission by optical rectification. However, the efficiency of THz generation in ZnTe is usually degraded by factors such as harmonic generation, two-photon absorption, and free-carrier absorption. In this work, we first report the effect of micro-scale Te inclusions in ZnTe on the local area THz emission. Unusual THz time-domain waveforms emitted from Te inclusions are observed to last longer than 400 ps, which is attributed to the photocurrent surge (PS) effect. This Te inclusion-associated THz radiation is proved consistent with the incident power and polarization angle dependences. Simultaneously, the Te inclusions embedded in the ZnTe matrix are visualized by laser THz emission microscopy (LTEM). According to the LTEM images, the driving force of the carrier movement is revealed, which is ascribed to the heterojunction field composed of Te and p-ZnTe. Our results not only afford a comprehensive understanding of the THz characteristics of Te inclusions but also put forward LTEM as a promising non-contact evaluation method for evaluating the bulk defects in semiconductors.

DOI： 10.1063/5.0045266

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Three-dimensional (3D) complementary metal-oxide-semiconductor (CMOS) integration could enable device scaling beyond the limits of conventional 2D CMOS technology. Such integration requires vertical electrical connections that pass through silicon substrates and interconnect stacked chips. The fabrication of these through-silicon vias (TSVs) creates new challenges in metrology, including the characterization of the thin isolation film deposited on the sidewalls of the TSVs and thickness characterization for wafer thinning. Here, we show that laser-induced terahertz emission microscopy can be used to characterize TSVs. Terahertz emission is observed through the excitation of a transient electric dipole in the depletion field of the TSV using femtosecond laser pulses. The detected terahertz waveform provides information about the local depletion field and thus structural information about the isolation film. By performing a time-of-flight measurement of the terahertz pulse, we can also extract the silicon wafer thickness.Laser-induced terahertz emission, and time-of-flight measurements of the terahertz pulse, can be used to non-invasively characterize through-silicon vias, which are required for three-dimensional CMOS integration.

DOI： 10.1038/s41928-021-00559-z

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<title>Abstract</title>Powerful, broadband terahertz (THz) pulses and its application attract an exponential growth of interests. Dual-color laser filamentation in gases is one of the promising THz sources because of the scalability of the THz energy and wavelength with input parameters. But the additional phase induced by the nonlinearities associated with high intensities cannot be neglected because it may result in modulation of the THz waves. We investigate the influences of the infrared pump energy and air dispersion on the terahertz generation in dual-color laser filament. We observe that optimum dual-color laser relative phase of the THz generation undergoes a linear shift with increasing pump energy due to the intensity-induced refractive index change. This phase shift is verified by the spectral broadening of a two-color laser affected by the same mechanism. The result improves our understanding of the theoretical framework for a higher power THz source.

DOI： 10.1038/s41598-020-80105-7

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Other Link： http://www.nature.com/articles/s41598-020-80105-7

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Terahertz waves have attracted great attention in biomolecule research because of the fact that they cover the range of energy levels of weak interactions, skeleton vibrations, and dipole rotations during inter- and intramolecular interactions in biomacromolecules. In this study, we validated the feasibility of employing terahertz time-domain spectroscopy (THz-TDS) for the nondestructive and label-free monitoring of protein digestion. The acid protease, pepsin, was used at its optimal pH to hydrolyze bovine serum albumin. Correspondingly, the control group experiment was also conducted by adjusting the pH value to inactivate pepsin. The progress of these two experiments was tracked by a compact commercial THz-TDS for 1 h. On one hand, the reaction-time-dependent absorption coefficient was calculated, and a direct absorption coefficient analysis was completed. The results indicate that protein hydrolysis can be easily monitored over time by focusing on the variation tendency of the absorption coefficient from a macroscopic perspective. On the other hand, we explored the use of the Debye model to analyze the dielectric properties of the solution during protein hydrolysis. The results of the Debye analysis prove that it is possible to investigate in detail the microscopic dynamics of biomacromolecule solutions at the molecular level by THz-TDS. Our research examined the process of protein hydrolysis by a combination of absorption spectra and Debye analysis and demonstrated that terahertz spectroscopy is a powerful technology for the investigation of biomolecular reactions, with potential applications in variety of fields.

DOI： 10.1016/j.bpj.2020.11.003

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

Imaging with terahertz (THz) waves has been expected as a non-invasive/non-staining visualization tool for breast cancer margins during surgeries. Breast cancer is a generic name for a heterogeneous lesion comprising invasive adenocarcinoma, in situ adenocarcinoma, most frequently in the form of ductal carcinoma in situ (DCIS) and benign tissues. Until now, THz imaging has focused on invasive adenocarcinoma; however, THz analysis of DCIS has hardly been performed. One of the reasons is that the size of an individual DCIS lesion, ranging from 50 to 500 mu m, is typically much smaller than that of an invasive carcinoma. This makes it difficult to identify these lesions by THz imaging, which has only a diffraction-limited spatial resolution of several millimeters. To overcome this drawback, we have developed a scanning point terahertz source (SPoTS) microscope with a resolution of 20 mu m, in which a near-infrared-pump-laser-induced two-dimensionally-scannable point THz source ((THz) phi(Pump)) generated in a GaAs crystal contacts a sample. In this study, utilizing this state-of-the-art microscope, we mainly performed THz near-field transmission imaging of a paraffin-embedded human breast cancer sample containing invasive carcinoma and DCIS, as a preliminary study. Consequently, for the first time, we succeeded in clearly visualizing a DCIS lesion of similar to 500 mu m in the THz images. It was also found that the THz attenuation by DCIS was higher than that by invasive ductal carcinoma. Furthermore, also in a reflection-mode measurement, we successfully obtained a similar outcome to the above transmission-mode one. These results can be caused by the interaction between the THz waves and the cellular density, indicating that SPoTS microscopy may be suitable for DCIS diagnosis.

DOI： 10.1088/2515-7647/abbcda

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Other Link： https://iopscience.iop.org/article/10.1088/2515-7647/abbcda/pdf

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI AG  

Although gallium oxide Ga2O3 is attracting much attention as a next-generation ultrawide bandgap semiconductor for various applications, it needs further optical characterization to support its use in higher-performance devices. In the present study, terahertz (THz) emission spectroscopy (TES) and laser THz emission microscopy (LTEM) are applied to Sn-doped, unintentionally doped, and Fe-doped β-Ga2O3 wafers. Femtosecond (fs) laser illumination generated THz waves based on the time derivative of the photocurrent. TES probes the motion of ultrafast photocarriers that are excited into a conduction band, and LTEM visualizes their local spatiotemporal movement at a spatial and temporal resolution of laser beam diameter and a few hundred fs. In contrast, one observes neither photoluminescence nor distinguishable optical absorption for a band-to-band transition for Ga2O3. TES/LTEM thus provides complementary information on, for example, the local mobility, surface potential, defects, band bending, and anisotropic photo-response in a noncontact, nondestructive manner. The results indicated that the band bends downward at the surface of an Fe-doped wafer, unlike with an n-type wafer, and the THz emission intensity is qualitatively proportional to the product of local electron mobility and diffusion potential, and is inversely proportional to penetration depth, all of which have a strong correlation with the quality of the materials and defects/impurities in them.

DOI： 10.3390/photonics7030073

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE RESEARCH  

© 2020, The Author(s). Gallium nitride (GaN) is a promising wide-bandgap semiconductor, and new characterization tools are needed to study its local crystallinity, carrier dynamics, and doping effects. Terahertz (THz) emission spectroscopy (TES) is an emerging experimental technique that can probe the ultrafast carrier dynamics in optically excited semiconductors. In this work, the carrier dynamics and THz emission mechanisms of GaN were examined in unintentionally doped n-type, Si-doped n-type, and Mg-doped p-type GaN films. The photocarriers excited near the surface travel from the excited-area in an ultrafast manner and generate THz radiation in accordance with the time derivative of the surge drift current. The polarity of the THz amplitude can be used to determine the majority carrier type in GaN films through a non-contact and non-destructive method. Unique THz emission excited by photon energies less than the bandgap was also observed in the p-type GaN film.

DOI： 10.1038/s41598-020-71728-x

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Optical Society  

© 2020 Chinese Laser Press We have fabricated low-temperature grown GaAs (LT-GaAs)-based plasmonic photoconductive antennas by RF sputtering of Au nanoparticles and have evaluated their terahertz detection properties. Localized surface plasmon resonance enhances the electric fields near the surface and increases the optical absorption of nanoparticles. The resonance frequency depends on the density of electrons, the effective electron mass, and the size and shape of the nanoparticles. Therefore, we tried to develop a high-sensitivity LT-GaAs photoconductive detector (PCD), which is effective over a wide range of wavelengths, by RF sputtering of Au nano-islands with a variety of aspect ratios from 1.2 to 5.1 on the dipole gap region of the PCD. As a result, we succeeded in increasing the sensitivity by 29% and 40% in the amplitude of observed terahertz pulse for 800 nm and 1560 nm femtosecond laser excitations, respectively.

DOI： 10.1364/PRJ.395517

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Authorship：Lead author,　Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We derive simple formulas to explain terahertz (THz) emission from semiconductor surfaces excited by a femtosecond (fs) laser. Femtosecond optical pulses with energies larger than the bandgap create photocarriers that travel and generate THz radiation, according to the time derivative of the photocurrent. By assuming that only electrons traveling in an ultrafast time scale, less than a few hundred fs, contribute to THz radiation, one can obtain simple expressions for the emission originating from the photocarrier drift accelerated with a built-in field or from the photocarrier diffusion. The emission amplitude of the former is in proportion with the electron mobility, the Schottky-barrier height, and the laser intensity and one of the latter with the laser intensity and diffusion coefficient squared. We also discuss the formula for emission from metal-insulator-semiconductor structures. The derived expressions are useful in understanding the THz emission properties observed by a laser THz emission microscope (LTEM), bringing the LTEM into real applications in the field of semiconductor research and development.

DOI： 10.1063/5.0005623

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

DOI： 10.1021/acs.nanolett.9b05082

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Transition metal oxides (TMOs) are among the most contemporary topics in condensed matter research. For over the past three decades, the most astounding discoveries of high temperature superconductivity, colossal magnetoresistance, multiferroics, topological matter, and several other novel electronic and magnetic phases have been realized and hosted in the TMOs. Almost simultaneously and specifically during the past 5 years, the implementation of terahertz (THz) spectroscopy to unveil the complexity of such materials has delivered unprecedented success. THz spectroscopy extracts material parameters such as dynamic conductivity, dielectric constant, and magnetic permeability in spectral range of 1-20 meV. This imparts the capability to probe a variety of quasiparticles emerging from the electrodynamics of collective, bound and free charge carriers, and complex spin orders, which is the key to decode the complex phases of TMOs class of quantum matter. Herein, the THz spectroscopic investigations of quasiparticle electrodynamics of correlated TMOs are reviewed. Several long-standing issues are resolved and there is enormous potential for a lot more to offer, emphasizing the growing indispensability of THz technology in condensed matter.

DOI： 10.1002/adom.201900958

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.201900958

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

During the past two decades, terahertz (THz) science and technology have rapidly expanded in all fundamental and applied aspects of physical, chemical, and biological sciences. In physical sciences, these developments have been twofolds: i) the use of THz technology in understanding the complexities of materials and ii) the exploration of new THz functionalities of emerging materials for further advancement of THz technology. Here, the THz emission spectroscopy and the ultrafast functionality of three pillars of electronic and magnetic condensed matter systems that emerged in the following order: high-temperature superconductors, colossal magnetoresistive manganites, and magnetoelectric multiferroics are reviewed. Emission functionality refers to the emitted THz radiation that carries the information of the underlying functional property of the material such that the overall effect evolves as the THz decoding of the information in a noninvasive manner and on an ultrafast timescale.

DOI： 10.1002/adom.201900892

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.201900892

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We developed a scanning laser terahertz (THz) near-field reflection imaging system using a fiber-coupled femtosecond laser source for high spatial resolution measurements without any sub-wavelength probes. In this system, THz wave pulses generated at the laser focusing spot in a non-linear optical crystal interact with a sample set on the crystal. Using this system, we obtained the THz reflection images of metallic structures with a resolution of similar to lambda(THz)/34 (lambda(THz) = 680 mu m) and also observed paraffin-embedded human breast tissue of several 10 mu m in size. These results promise that this system can be suitable for industrial applications which require micron-scale resolution. (C) 2019 The Japan Society of Applied Physics

DOI： 10.7567/1882-0786/ab4ddf

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/1.5083674

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Photoconductive detectors (PCDs) with wide dipole electrodes were fabricated on low temperature-grown GaAs for observation of low frequency terahertz electromagnetic waves. The screening effect in PCDs caused by the space charge of photoexcited electron-hole pairs brings poor sensitivity in detection. This can be effectively reduced by widening the dipole electrodes in PCDs. The width dependence of terahertz (THz) wave detection properties of PCDs showed that not only the sensitivity but also the signal-to-noise ratio can be fairly improved by increasing the width, especially at frequencies lower than 1.2 THz. Published under license by AIP Publishing.

DOI： 10.1063/1.5080750

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Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

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Authorship：Last author   Language：Japanese   Publishing type：Research paper (scientific journal)  

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

Biosensing with terahertz (THz) waves has received large amounts of attention due to its potential to detect the functional expression of biomolecules in a label-free fashion. However, many practical challenges against the diffraction limit of THz waves and the strong absorption of THz waves into polar solvents still remain in the development of compact biosensors. Here, we present a non-linear, optical, crystal-based THz-microfluidic chip with a few arrays of asymmetric meta-atoms, an elementary unit of metamaterials, for the measurement of trace amounts of solution samples. A near-field THz emission source, that is locally generated in the process of optical rectification at a fs (femtosecond) laser irradiation spot, induces a sharp Fano resonance and modifies the resonance frequency of the meta-atoms when the channel is filled with solution samples of different concentrations. Using this chip, we successfully detected minute changes in the concentration of trace amounts of mineral water and aqueous sugar solutions by monitoring the shift in the resonance frequency. A higher detectable sensitivity of 1.4 fmol of solute in a 128 pL volume of solution was achieved. This was an improvement of one order of magnitude in the sensitivity compared to our previous experiment.

DOI： 10.3390/photonics6010012

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

In the present work, an opportunity of nearly single-cycle THz pulse generation in aperiodically poled lithium niobate (APPLN) crystal is studied. A radiating antenna model is used to simulate the THz generation from chirped APPLN crystal pumped by a sequence of femtosecond laser pulses with chirped delays (m = 1, 2, 3 horizontal ellipsis ) between adjacent pulses. It is shown that by appropriately choosing Delta t(m), it is possible to obtain temporal overlap of all THz pulses generated from positive (or negative) domains. This results in the formation of a nearly single-cycle THz pulse if the chirp rate of domain length delta in the crystal is sufficiently large. In the opposite case, a few cycle THz pulses are generated with the number of the cycles depending on delta. The closed-form expression for the THz pulse form is obtained. The peak THz electric field strength of 0.3 MV/cm is predicted for APPLN crystal pumped by a sequence of laser pulses with peak intensities of the separate pulse in the sequence of about 20 GW/cm(2). By focusing the THz beam and increasing the pump power, the field strength can reach values in the order of few MV/cm.

DOI： 10.3390/photonics6010009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The concept of the tricolor superstructure (TCS), which is a triple-layer stack structure containing two types of high dielectric constant (high-k) layers (designated HK1 and HK2) and a SiO2 layer, is proposed to control the moment and the polarity of the interface dipole layer that are induced at the high-k/SiO2 interfaces. The interface dipole layer is formed by oxygen ion migration from the layer with higher oxygen areal density (sigma) to that with lower r. When the two high-k materials are selected with the order of sigma(HK1)> sigma(SiO2)> sigma(HK2) in a SiO2/HK2/HK1/SiO2 TCS, the dipole directions of the interface dipole layers at the SiO2/HK2 and the HK1/SiO2 interfaces are aligned. Additionally, in the transposed SiO2/HK1/HK2/SiO2 TCS, the total polarity is reversed. The concept is demonstrated using Al2O3 and Y2O3 layers because they offer the order of sigma(Al2O3)> sigma(SiO2)> sigma(Y2O3). The two stacking sequence samples composed of SiO2/Y2O(3)/Al2O3/SiO2 and SiO2/Al2O3/Y2O3/SiO2 that were fabricated using superlattice technique by pulsed laser deposition obviously show opposite dipole polarities. Increasing repetition of the deposited TCS unit also causes the dipole moments to increase systematically. The TCS technique enables control of the properties of the interface dipole layer at high-k/SiO2 interfaces in amorphous systems. Published by AIP Publishing.

DOI： 10.1063/1.5034494

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We present a nonlinear optical crystal (NLOC)-based terahertz (THz) microfluidic chip with a few arrays of split ring resonators (SRRs) for ultra-trace and quantitative measurements of liquid solutions. The proposed chip operates on the basis of near-field coupling between the SRRs and a local emission of point like THz source that is generated in the process of optical rectification in NLOCs on a sub-wavelength scale. The liquid solutions flowing inside the microchannel modify the resonance frequency and peak attenuation in the THz transmission spectra. In contrast to conventional bio-sensing with far/near-field THz waves, our technique can be expected to compactify the chip design as well as realize high sensitive near-field measurement of liquid solutions without any high-power optical/THz source, near-field probes, and prisms. Using this chip, we have succeeded in observing the 31.8 fmol of ion concentration in actual amount of 318 pl water solutions from the shift of the resonance frequency. The technique opens the door to microanalysis of biological samples with THz waves and accelerates development of THz lab-on-chip devices. (c) 2018 Author(s).

DOI： 10.1063/1.5007681

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We present the direct measurements of terahertz meta-atoms, an elementary unit of metamaterials, by using locally generated terahertz waves in the near-field region. In contrast to a conventional far-field terahertz spectroscopy or imaging, our technique features the localized emission of coherent terahertz pulses on a sub-wavelength scale, which has a potential for visualizing details of dynamics of each meta-atom. The obtained data show the near-field coupling among the meta-atoms and the impact of the electric field distribution from the excited meta-atom to neighbor meta-atoms. The observable LC resonance response is enhanced with an increase of numbers of meta-atoms. Furthermore, our approach also has a potential for visualizing the individual mode of meta-atom at different terahertz irradiation spots. These data can help us to understand the important role of the meta-atom in metamaterials and develop the novel terahertz components and devices such as active terahertz metamaterial and compact, high-sensitive bio-sensor devices.

DOI： 10.1007/s10762-017-0417-7

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DOI： 10.1007/s10762-017-0429-3

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Parallel plate waveguide (PPWG) terahertz (THz) time domain spectroscopy (TDS) is a powerful tool to investigate the properties of thin and low conductive materials. In this Letter, we determine the lower limit of detection of the PPWG-THz-TDS approach. We provide a closed-form expression of the minimal measurable conductivity by the system. The experimental results of amorphous YBa2Cu3O7-b films indicate that the factor limiting the spectroscopic modality is the waveguide device misalignment. On the other hand, the expression of the minimal detectable conductivity provides a clear scheme of optimization by increasing the waveguide length and therefore enhancing the sensitivity of the system. (C) 2017 Optical Society of America

DOI： 10.1364/OL.42.003056

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We demonstrate in this paper the tunable electromagnetically induced transparency (EIT) made from a superconducting (SC) niobium nitride (NbN) film induced by an intense terahertz (THz) field. As the variation of the incident THz field alters the intrinsic ohmic loss of the SC NbN film, the field-dependent transmittance is observed. To elaborate the role of the bright and dark modes, a hybrid coupling model is introduced to fit the experimental transmission spectra and extract the characteristic parameters of each mode. It is shown that the resonator for the bright mode is altered greatly due to strong direct coupling to the incident intense THz field, whereas the dark mode resonator has little interaction with the incident THz field via a weak near-filed coupling to the brightmode resonator. This implies that we can partially control a mode or a part of metamaterial by introducing the intense THz field, which offers an effective manner to selectively control the electromagnetic property of the metamaterial. This work may bring many potential applications for the tunable EIT-like metamaterial. Published by AIP Publishing.

DOI： 10.1063/1.4985618

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Adsorption of gas molecules on the surface of atomically layered two-dimensional (2D) materials, including graphene and transition metal dichalcogenides, can significantly affect their electrical and optical properties. Therefore, a microscopic and quantitative understanding of the mechanism and dynamics of molecular adsorption and desorption has to be achieved in order to advance device applications based on these materials. However, recent theoretical calculations have yielded contradictory results, particularly on the magnitude of the adsorption energy. Here, we have experimentally determined the adsorption energy of oxygen molecules on graphene and 2D tungsten disulfide using temperature-programmed terahertz (THz) emission microscopy (TPTEM). The temperature dependence of THz emission from InP surfaces covered with 2D materials reflects the change in oxygen concentration due to thermal desorption, which we used to estimate the adsorption energy of oxygen molecules on graphene (similar to 0.15 eV) and tungsten disulphide (similar to 0.24 eV). Furthermore, we used TPTEM to visualize relative changes in the spatial distribution of oxygen molecules on monolayer graphene during adsorption and desorption. Our results provide much insight into the mechanism of molecular adsorption on the surface of 2D materials, while introducing TPTEM as a novel and powerful tool for molecular surface science.

DOI： 10.1038/s41598-017-01883-1

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Surface polar states in m-plane GaN wafers were studied using a laser terahertz (THz) emission microscope (LTEM). Femtosecond laser illumination excites THz waves from the surface due to photocarrier acceleration by local spontaneous polarization and/or the surface built-in electric field. The m-plane, in general, has a large number of unfavorable defects and unintentional polarization inversion created during the regrowth process. The LTEM images can visualize surface domains with different polarizations, some of which are hard to visualize with photoluminescence mapping, i.e., non-radiative defect areas. The present study demonstrates that the LTEM provides rich information about the surface polar states of GaN, which is crucial to improve the performance of GaN-based optoelectronic and power devices. (C) 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license.

DOI： 10.1063/1.4979511

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Using laser terahertz emission microscopy, we measured laser-excited terahertz (THz) emission from silicon wafers with silicon-oxide passivation layers, revealing a strong correlation between the THz waveform and the surface potential. The surface potential was electrically tuned by a semitransparent top electrode disc and evaluated by measuring capacitance-voltage characteristics. The waveform changed with external bias and inverted near the flatband voltage, and changes appeared in the peak amplitude were similar to the capacitance-voltage characteristics. These results indicate that by analyzing the waveform of laser-excited THz emission generated by laser terahertz emission microscopy, we could quantitatively measure and map the internal field of surface band bending in semiconductors. Published by AIP Publishing.

DOI： 10.1063/1.4980847

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

IV-characteristics of stacks of two inductively interacting long Josephson junctions with the homogeneous and inhomogeneous distributions of critical currents were investigated numerically. It was assumed that the inhomogeneous linear distribution of critical currents along the junction was created by heating of one end of the stack. Even zero-field steps were found in the IV-curve of the stack with the homogeneous distribution of critical currents, whereas odd zero-field steps appeared in the IV-curve of the stack with the heated end. Due to the inductive interaction between junctions in a stack of two junctions, each of the zero-field steps splits into two steps which correspond to frequencies of collective excitations in the system. Strong coherent emission was found at the step which corresponds to the frequency of in-phase oscillations.

DOI： 10.1088/0953-2048/30/1/014004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Herein, terahertz (THz) time domain spectroscopy is used to measure the complex conductivity of semi-insulating CoFe2O4 nanoparticles (NPs) and nano-hollow spheres (NHSs) with different diameters ranging from 100 to 350 nm having a nanocrystalline shell thickness of 19 to 90 nm, respectively. Interestingly, the magnitude of conductivity for CoFe2O4 NPs and NHSs of same average diameter (similar to 100 nm) for a given frequency of 0.3 THz is found to be 0.33 S/m and 9.08 S/m, respectively, indicating that the hollow structure exhibits greater THz conduction in comparison to its solid counterpart. Moreover, THz conductivity can be tailored by varying the nano-shell thickness of NHSs, and a maximum conductivity of 15.61 S/m is observed at 0.3 THz for NHSs of average diameter 250 nm. A detailed study reveals that thermally activated polaronic hopping plays the key role in determining the electrical transport property of CoFe2O4 nanostructures, which is found to solely depend on their magnitude of THz absorptivity. The non-Drude conductivity of all CoFe2O4 nanostructures is well described by the Polaron model instead of the Drude-Smith model, which is relevant for backscattering of free electrons in a nanostructured material. The Polaron model includes intra-particle and interparticle polaronic conductivities for closely spaced magnetic nanostructures and provides a mean free path of 29nm for CoFe2O4 NPs of diameter 100nm, which is comparable with its average crystallite size, indicating the applicability of the developed model for nanomaterials where charge transport is determined by polaronic hopping. Finally, we have demonstrated the morphology and size dependent magnetic measurements of ferrimagnetically aligned CoFe2O4 nanostructures through a vibrating sample magnetometer in the temperature range of 80-250 K, revealing that the disordered surface spin layer of nanostructures significantly controls their magnetism. Published by AIP Publishing.

DOI： 10.1063/1.4967923

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

We have studied coherent terahertz (THz) emission from graphene-coated surfaces of three different semiconductors-InP, GaAs, and InAs-to provide insight into the influence of O-2 adsorption on charge states and dynamics at the graphene/semiconductor interface. The amplitude of emitted THz radiation from graphene-coated InP was found to change significantly upon desorption of O-2 molecules by thermal annealing, while THz emission from bare InP was nearly uninfluenced by O-2 desorption. In contrast, the amount of change in the amplitude of emitted THz radiation due to O-2 desorption was essentially the same for graphene-coated GaAs and bare GaAs. However, in InAs, neither graphene coating nor O-2 adsorption/desorption affected the properties of its THz emission. These results can be explained in terms of the effects of adsorbed O-2 molecules on the different THz generation mechanisms in these semiconductors. Furthermore, these observations suggest that THz emission from graphene-coated semiconductors can be used for probing surface chemical reactions (e.g., oxidation) as well as for developing O-2 gas sensor devices.

DOI： 10.1007/s10762-016-0301-x

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Other Link： http://link.springer.com/article/10.1007/s10762-016-0301-x/fulltext.html

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We observed photoexcited carrier responses in solar cells excited by femtosecond laser pulses with spatial and temporal resolution using an optical pump-terahertz emission probe technique. We visualized the ultrafast local variation of the intensity of terahertz emission from a polycrystalline silicon solar cell using this technique and clearly observed the change in signals between a grain boundary and the inside of a grain in the solar cell. Further, the time evolution of the pump-probe signals of the polycrystalline and monocrystalline silicon solar cells was observed, and the relaxation times of photoexcited carriers in the emitter layers of crystalline silicon solar cells were estimated using this technique. The estimated relaxation time was consistent with the lifetime of the Auger recombination process that was dominant in heavily doped silicon used as an emitter layer for the silicon solar cells, which is difficult to obtain with photoluminescence method commonly used for the evaluation of solar cells.

DOI： 10.1007/s10762-015-0233-x

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

We propose dynamic terahertz (THz) emission microscopy (DTEM) to visualize temporal-spatial dynamics of photoexcited carriers in electronic materials. DTEM utilizes THz pulses emitted from a sample by probe pulses irradiated after pump pulse irradiation to perform time-resolved two-dimensional mapping of the THz pulse emission, reflecting various carrier dynamics. Using this microscopy, we investigated carrier dynamics in the gap region of low-temperature-grown GaAs and semi-insulating GaAs photoconductive switches of the identical-dipole type. The observed DTEM images are well explained by the change in the electric potential distribution between the electrodes caused by the screening effect of the photoexcited electron-hole pairs. (C) 2016 Chinese Laser Press

DOI： 10.1364/PRJ.4.000208

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

As new designs of metamaterials rapidly emerge, methods of characterizing their fundamental electromagnetic properties become increasingly important. Here, we utilize the parallel plate waveguide associated with terahertz time-domain spectroscopy experiments to analyze the coupling of terahertz radiation to ultrathin electric split-ring resonators located halfway between the waveguide plates. Our observations determine that the magnetic response dominates across the frequency range of the system. The experimental results are confirmed by simulations, emphasizing the usefulness of the proposed approach for further investigations of magnetic coupling in metamaterials in the terahertz regime. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/APEX.9.032002

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

As novel classes of two-dimensional (2D) materials and heterostructures continue to emerge at an increasing pace, methods are being sought for elucidating their electronic properties rapidly, non-destructively, and sensitively. Terahertz (THz) time-domain spectroscopy is a well-established method for characterizing charge carriers in a contactless fashion, but its sensitivity is limited, making it a challenge to study atomically thin materials, which often have low conductivities. Here, we employ THz parallel-plate waveguides to study monolayer graphene with low carrier densities. We demonstrate that a carrier density of similar to 2 x 10(11) cm(-2), which induces less than 1% absorption in conventional THz transmission spectroscopy, exhibits similar to 30% absorption in our waveguide geometry. The amount of absorption exponentially increases with both the sheet conductivity and the waveguide length. Therefore, the minimum detectable conductivity of this method sensitively increases by simply increasing the length of the waveguide along which the THz wave propagates. In turn, enabling the detection of low-conductivity carriers in a straightforward, macroscopic configuration that is compatible with any standard time-domain THz spectroscopy setup. These results are promising for further studies of charge carriers in a diverse range of emerging 2D materials. (C) 2016 Optical Society of America

DOI： 10.1364/OE.24.003885

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Language：Japanese   Publisher：The Japan Society for Precision Engineering  

DOI： 10.2493/jjspe.82.225

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEICE-INST ELECTRONICS INFORMATION COMMUNICATIONS ENG  

Up to now, broadband THz time-domain system has been developed and widely used for THz inspection system; however for many THz devices for THz band wireless communication, narrow-band system would be preferred rather than typical broadband system. In this work we established a narrowband and time-domain THz radiation and detection system and characterized uncooled microbolometer-based THz imagers using that system. The central frequency of generated narrowband THz wave was 850 GHz. This system enables simultaneous measurement of pulse energy and waveform of THz pulse using a superconducting transition edge sensor for measuring energy and electro-optic sampling for measuring THz waveform. We used this system to evaluate the performance of uncooled THz imagers; IRV-T0831 and T0832 from NEC. Noise equivalent power (NEP) of approximately 0.22 pW/Hz(1/2) was achieved in case of T0832 at less than 1 THz which is lower than NEP value of previous reports.

DOI： 10.1587/transele.E98.C.1128

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

Development of techniques for characterization of extremely thin films is an important challenge in terahertz (THz) science and applications. Spectroscopic measurements of materials on the nanometer scale or of atomic layer thickness (2D materials) require a sufficient terahertz wave-matter interaction length, which is challenging to achieve in conventional transmission geometry. Waveguide-based THz spectroscopy offers an alternative method to overcome this problem. In this paper, we investigate a new parallel-plate waveguide (PPWG) technique for measuring dielectric properties of ultrathin gold films, in which we mount the thin film sample at the center of the waveguide. We discuss a model of THz dielectric parameter extraction based on waveguide theory and analyze the response of thin films for both transverse magnetic (TM) and transverse electric (TE) waveguide modes. In contrast to other waveguide methods, our approach enables comparison of the material response with different electromagnetic field distributions without significantly changing the experimental setup. As a result, we demonstrate that TE modes have a better sensitivity to the properties of the thin film. For prototype test samples, optical parameters extracted using our method are in good agreement with literature values.

DOI： 10.1007/s10762-015-0194-0

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A laser terahertz emission microscope (LTEM) can be used for noncontact inspection to detect the waveforms of photoinduced terahertz emissions from material devices. In this study, we experimentally compared the performance of LTEM with conventional analysis methods, e. g., electroluminescence (EL), photoluminescence (PL), and laser beam induced current (LBIC), as an inspection method for solar cells. The results showed that LTEM was more sensitive to the characteristics of the depletion layer of the polycrystalline solar cell compared with EL, PL, and LBIC and that it could be used as a complementary tool to the conventional analysis methods for a solar cell. (C) 2015 Author(s).

DOI： 10.1063/1.4935913

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Superconductor is a compelling plasmonic medium at terahertz frequencies owing to its intrinsic low Ohmic loss and good tuning property. However, the microscopic physics of the interaction between terahertz wave and superconducting plasmonic structures is still unknown. In this paper, we conducted experiments of the enhanced terahertz transmission through a series of superconducting NbN subwavelength hole arrays, and employed microscopic hybrid wave model in theoretical analysis of the role of hybrid waves in the enhanced transmission. The theoretical calculation provided a good match of experimental data. In particular, we obtained the following results. When the width of the holes is far below wavelength, the enhanced transmission is mainly caused by localized resonance around individual holes. On the contrary, when the holes are large, hybrid waves scattered by the array of holes dominate the extraordinary transmission. The surface plasmon polaritions are proved to be launched on the surface of superconducting film and the excitation efficiency increases when the temperature approaches critical temperature and the working frequency goes near energy gap frequency. This work will enrich our knowledge on the microscopic physics of extraordinary optical transmission at terahertz frequencies and contribute to developing terahertz plasmonic devices.

DOI： 10.1038/srep15588

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Wide-gap semiconductors have received significant attention for their advantages over existing semiconductors in energy-efficient power devices. To realize stable and reliable wide-gap semiconductor devices, the basic physical properties, such as the electric properties on the surface and at the interface, should be revealed. Here, we report visualization of terahertz (THz) emission from the surface of GaN, which is excited by ultraviolet femtosecond laser pulses. We found that the THz emission is enhanced by defects related to yellow luminescence, and this phenomenon is explained through the modification of band structures in the surface depletion layer owing to trapped electrons at defect sites. Our results demonstrate that the surface potential in a GaN surface could be detected by laser-induced THz emission. Moreover, this method enables feasible evaluation of the distribution of non-radiative defects, which are undetectable with photoluminescence, and it contributes to the realization normally-off GaN devices.

DOI： 10.1038/srep13860

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Herein, we investigate the origin of enhanced absorption and complex conductivity of magnetite (Fe3O4) nano-hollow spheres (NHSs) in contrast to its nanoparticles (NPs) configuration in the frequency range 0.4-2.0 THz. The maximum absorption for NHSs and NPs of the same average diameter (similar to 100 nm) are found to be 246.27 and 48.35 cm(-1) at 1.8 THz, respectively. A detailed study suggests that the multiple resonance peaks in the absorption spectra are due to low frequency acoustic vibrational phonon modes of Fe3O4 nanostructures. Moreover, we demonstrate that the magnitude of total absorption can be tailored by varying the shell thickness of NHSs. It is found to increase with increasing shell thickness, and attain a maximum value of 498.5 cm(-1) for the NHSs of average diameter 350 nm at 1.8 THz. The invariance of frequency dependent magnetic permeability points out that the absorption is basically due to dielectric loss instead of magnetic loss. The enhanced THz conductivity of Fe3O4 NHSs, as compared to NPs is described in light of thermally activated polaronic hopping which is found to increase with increasing THz absorption. Finally, the size dependent THz conductivity of NHSs confirms its sole dependence on the magnitude of THz absorptivity.

DOI： 10.1088/0022-3727/48/24/245301

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

We present the result of incorporating a heavily Si-doped InAs reflection layer into a terahertz emitting, InAs thin film based, structure. We demonstrate the increase of the radiated electric field intensity relative to a typical, n-type InAs bulk reference crystal using terahertz time domain spectroscopy (THz-TDS). With our approach more than a 2 fold increase in the electric field can be obtained. We compare our experimental findings with values obtained from calculations in the Drude model to obtain good agreement.

DOI： 10.1007/s10762-015-0147-7

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Other Link： http://link.springer.com/article/10.1007/s10762-015-0147-7/fulltext.html

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2015 Materials Research Society. We applied laser THz emission spectroscopy to study the effects of monolayer graphene on the THz emission from InAs. THz emission from graphene/InAs varies linearly with the laser excitation power in the low-intensity excitation regime. We found that unlike in graphenc/SI-InP junctions, graphene and O2 adsorbates on graphene have no significant effect on the THz emission from graphene/InAs junctions because the THz radiation mechanism in InAs is by the photo-Dember effect, whereas for S1-InP is by the surge current effect. There is also a slight enhancement in the THz emission from both bare InAs and graphene/InAs by UV illumination, which is probably due to the additional photoexcited carriers by UV that somehow enhances the photo-Dember field.

DOI： 10.1557/opl.2015.639

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

Nonlinear terahertz (THz) transmission through subwavelength hole array in superconducting niobium nitride (NbN) film is experimentally investigated using intense THz pulses. The good agreement between the measurement and numerical simulations indicates that the field strength dependent transmission mainly arises from the nonlinear properties of the superconducting film. Under weak THz pulses, the transmission peak can be tuned over a frequency range of 145 GHz which is attributed to the high kinetic inductance of 50nm-thick NbN film. Utilizing the THz pump-THz probe spectroscopy, we study the dynamic process of transmission spectra and demonstrate that the transition time of such superconducting plasmonic device is within 5 ps.

DOI： 10.1063/1.4898818

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The laser terahertz (THz) emission microscope (LTEM) is a unique inspection tool which can directly two-dimensionally map the THz pulse emission from a variety of electric materials and devices. Thus, two-dimensional mapping directly yields various physical information by visualizing the distributions of electric field, supercurrent, ferroelectric domain structures, magnetic fluxes, etc. Based on techniques created for conventional LTEM, we have developed a more highly functional laser THz imaging system, including a scanning laser THz (near-field) imaging system having the performance of high-speed and high-resolution, a scanning probe LTEM coupled with an atomic force microscope for high-resolution imaging, and a dynamic THz emission microscope to investigate the ultrafast carrier dynamics two-dimensionally in the sample. These systems offer diverse characteristics, making them suitable for various applications. We take a look at these systems and some typical applications.

DOI： 10.1088/0022-3727/47/37/374007

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We have deposited 1.0-mu m-thick BaTiO3 (BTO), SrTiO3 (STO), Ba0.5Sr0.5TiO3 (BST) thin films on MgO substrates by pulsed laser deposition technique and measured their dielectric properties by terahertz-time domain spectroscopy (THz-TDS) and an impedance analyzer with interdigital surface electrodes in the temperature range from 20 to 290 K. By comparing the real part of the dielectric constant at 1 MHz and 1 THz, we have estimated the temperature dependence of the orientational polarization. The results suggest that the orientational polarization emerges in the ferroelectric phase of (Ba,Sr)TiO3, and the concentration of orientational polarization components of BST exceeds 90% of the total below 150K, while it is about 30% at room temperature. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.53.09PD06

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Being an atomically thin material, graphene is known to be extremely susceptible to its environment, including defects and phonons in the substrate on which it is placed as well as gas molecules that surround it. Thus, any device design using graphene has to take into consideration all surrounding components, and device performance needs to be evaluated in terms of environmental influence. However, no methods have been established to date to readily measure the density and distribution of external perturbations in a quantitative and non-destructive manner. Here, we present a rapid and non-contact method for visualizing the distribution of molecular adsorbates on graphene semi-quantitatively using terahertz time-domain spectroscopy and imaging. We found that the waveform of terahertz bursts emitted from graphene-coated InP sensitively changes with the type of atmospheric gas, laser irradiation time, and ultraviolet light illumination. The terahertz waveform change is explained through band structure modifications in the InP surface depletion layer due to the presence of localized electric dipoles induced by adsorbed oxygen. These results demonstrate that terahertz emission serves as a local probe for monitoring adsorption and desorption processes on graphene films and devices, suggesting a novel two-dimensional sensor for detecting local chemical reactions.

DOI： 10.1038/srep06046

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS  

Terahertz wave-emission properties of a polysilicon solar cell excited by femtosecond laser pulses were visualized by laser terahertz emission microscopy (LTEM). The effects on the terahertz emissions of continuous-wave (cw) laser illumination of the solar cell at two wavelengths (808 and 365 nm) were examined under conditions of reverse and zero bias. The amplitude of the terahertz pulses in the presence of illumination was lower than that in its absence, and it decreased with increasing illumination power. The photovoltaic effect was smaller for cw laser illumination at the shorter wavelength. These phenomena can be explained in terms of screening of the electric field in the depletion layer of the p-n junction, as a result of the presence of photoexcited carriers. In addition, the LTEM images permitted the visualization of the crystalline grain structure of the solar cell and the dynamics of photocarriers, and they reflected the local electric field distribution in the cell. LTEM was therefore shown to be a promising technique for the evaluation and inspection of solar cells. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

DOI： 10.1117/1.OE.53.3.031204

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS  

Single human hairs using a scanning laser terahertz (THz) imaging system are evaluated. The system features near-field THz emission and far-field THz detection. A sample is set in the vicinity of a two-dimensional THz emitter, and an excitation laser beam is scanned over the emitter via a galvanometer. By detecting the transmitted THz wave pulses that are locally generated at the irradiation spots of the excitation laser, we can obtain the THz transmission image and the spectrum of the sample with imaging time of 47 s for 512 x 512 pixels and maximum resolution of similar to 27 mu m. Using the system, we succeeded in observing the specific features of single human hairs in both the THz transmittance spectra and transmission images; it was found that the THz transmittance spectrum of gray hair shows a tendency of increase while that of black hair shows a decrease with increasing frequency above 1.2 THz. We could also observe the change of the moisture retention in the hair, and it is found that cuticles play one of the important roles in keeping moisture inside the hair. Those obtained data indicate that our system can be useful for evaluating single human hairs and those kinds of microscale samples. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

DOI： 10.1117/1.OE.53.3.031205

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS  

Transmission-type terahertz time-domain spectroscopy is applied to evaluate crystallized lactose particle of size below 30 mu m, which is far too small compared to the wavelength of incident terahertz (THz)-wave. The THz-absorption spectrum of lactose is successfully deconvoluted by Lorentzian to two spectra with peaks at 17.1 cm(-1) (0.53 THz) and 45.6 cm(-1) (1.37 THz) derived from alpha-lactose monohydrate, and a spectrum at 39.7 cm(-1) (1.19 THz) from anhydrous beta-lactose after removal of the broad-band spectrum by polynomial cubic function. Lactose is mainly crystallized into alpha-lactose monohydrate from the supersaturated solution at room temperature with a small amount of anhydrous beta-lactose below 4%. The absorption feature is dependent on the crystallized particle size and the integrated intensity ratio of the two absorptions due to alpha-lactose monohydrate is correlated in linear for the size. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

DOI： 10.1117/1.OE.53.3.031203

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

DOI： 10.1002/adom.201300321

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

By measuring the detailed characteristics of terahertz-pulse (THz-pulse) radiation and applying THz time-domain spectroscopy, we investigate the generation mechanism of THz-pulse radiation from biased YBa2Cu3O7-delta (YBCO) thin films with various carrier concentrations. To vary the carrier concentration, the oxygen deficiency of the YBCO samples was systematically varied. With decreasing carrier concentration, the THz-pulse width increases and the response speed of supercarriers to illumination by femtosecond optical pulses tends to decrease. By examining the radiated waveforms, we clarify the influence of carrier doping not only on the relaxation process of quasiparticles but also on the excitation process. A suitably underdoped YBCO film results in maximum efficiency for generating THz radiation. This tendency is difficult to explain if we only consider the variation in transmittance, which has been the approach till now. Instead, we suggest that a variation in quasiparticle excitation efficiency due to carrier doping should also be considered.

DOI： 10.1007/s10762-013-0016-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Structured plasmonic metamaterial devices offer the design flexibility to be size scaled for operation across the electromagnetic spectrum and are extremely attractive for generating electromagnetically induced transparency and slow-light behaviors via coupling of bright and dark subwavelength resonators. Here, we experimentally demonstrate a thermally active superconductor-metal coupled resonator based hybrid terahertz metamaterial on a sapphire substrate that shows tunable transparency and slow light behavior as the metamaterial chip is cooled below the high-temperature superconducting phase transition temperature. This hybrid metamaterial opens up the avenues for designing micro-sized active circuitry with switching, modulation, and "slowing down terahertz light" capabilities. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4819389

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Recent progress in terahertz technology has enabled precise investigation of the ultrafast dynamics of excited carriers, nonequilibrium state and nonlinear response of superconductors, resulting in the proposal of novel optoelectronic device applications based on such ultrafast perturbation of supercarriers in the terahertz frequency region. In this paper, we focus on exploratory research in the field of superconductor terahertz science and technology, and present a review of superconducting terahertz sources and the response of superconductors excited by ultrashort electromagnetic pulses, including optical pulses and high-intensity THz pulses.

DOI： 10.1088/0953-2048/26/9/093002

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Authorship：Last author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The half-doped Pr0.5Sr0.5MnO3 (PSMO) manganite represents a unique stripe type of charge orbital order that induces transport and magnetic anisotropy. Low-energy charge dynamics in PSMO (110) epitaxial film were investigated along and across two dissimilar in-plane orthogonal axes, [1-10] and [001], by recording the complex refractive index in the energy range of 1-7 meV. We report a novel observation of a gradual crossover from Drude-like metallic conductivity to charge-density-wave (CDW)-like collective excitations as the polarized terahertz excitation field is swept across the orthogonal in-plane axes. This is a rare manifestation of a CDW mode in a highly conducting stripe-type charge-ordered (CO) system, which is fundamentally different from the CDW modes of checkerboard CO. Interestingly, the contrasting charge dynamics along orthogonal in-plane axes form an intrinsic electronic switch, apparently a consequence of the unique CO of PSMO.

DOI： 10.1103/PhysRevB.87.224421

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Plasmon induced transparency (PIT) could be realized in metamaterials via interference between different resonance modes. Within the sharp transparency window, the high dispersion of the medium may lead to remarkable slow light phenomena and an enhanced nonlinear effect. However, the transparency mode is normally localized in a narrow frequency band, which thus restricts many of its applications. Here we present the simulation, implementation, and measurement of a broadband PIT metamaterial functioning in the terahertz regime. By integrating four U-shape resonators around a central bar resonator, a broad transparency window across a frequency range greater than 0.40 THz is obtained, with a central resonance frequency located at 1.01 THz. Such PIT metamaterials are promising candidates for designing slow light devices, highly sensitive sensors, and nonlinear elements operating over a broad frequency range.

DOI： 10.1088/0957-4484/24/21/214003

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We present the nonlinear response of superconducting niobium nitride (NbN) thin film and NbN metamaterial with different thicknesses under intense terahertz pulses. For NbN thin film, nonlinearity emerges and superconductivity is suppressed with increasing incident terahertz electric field, and the suppression extent weakens as the film thickness increases from 15 to 50 nm. As the variation in intense terahertz fields alters the intrinsic conductivity in NbN, a consequent remarkable amplitude modulation in NbN metamaterial is observed due to the strong nonlinearity. Absorbed photo density in either NbN film or NbN metamaterial is estimated and used to understand the mechanism of nonlinear response. With a thicker NbN film element of 200 nm, the resonance of the metamaterial shows similar nonlinear modulation accompanied by a lower loss and a higher quality factor compared with a thinner NbN film element of 50 nm, which demonstrates the innovative implementation of strongly enhanced nonlinearity with thick superconducting film elements and the potential for novel applications using nonlinear metamaterial.

DOI： 10.1088/1367-2630/15/5/055017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study macroscopically aligned single-wall carbon nanotube arrays with uniform lengths via polarization-dependent terahertz and infrared transmission spectroscopy. Polarization anisotropy is extreme at frequencies less than similar to 100 cm(-1) with no sign of attenuation when the polarization is perpendicular to the alignment direction. The attenuation for both parallel and perpendicular polarizations increases with increasing frequency, exhibiting a pronounced and broad peak around 450 cm(-1) in the parallel case. We model the electromagnetic response of the sample by taking into account both radiative scattering and absorption losses. We show that our sample acts as an effective antenna due to the high degree of alignment, exhibiting much larger radiative scattering than absorption in the mid/far-infrared range. Our calculated attenuation spectrum clearly shows a non-Drude peak at similar to 450 cm(-1) in agreement with the experiment. DOI: 10.1103/PhysRevB.87.161401

DOI： 10.1103/PhysRevB.87.161401

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

We present a simple scheme of narrowband terahertz (THz) generation by optical rectification in the lithium niobate crystal covered by a binary phase mask. It is shown that a single-domain crystal illumination by spatiotemporal shaped fs-laser pulses is equivalent to the formation of a transversally patterned, quasi-phase-matching structure. Decrease of the optical beam size on the mask leads to an increase of the THz-wave linewidth from 17 GHz to a few THz. The frequency of the generation was tuned in the range of 0.4-1.0 THz by building images of the mask in the crystal with various magnifications. Application results of the presented THz source for measuring transmittance of the superconducting NbN thin film in the 4.2-15 K temperature range are also presented. (C) 2013 Optical Society of America

DOI： 10.1364/OL.38.000953

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We investigate the nonlinear effect of a planar superconducting metamaterial made from niobium nitride (NbN) at terahertz frequencies. As the variation of the incident intense terahertz field alters the intrinsic conductivity in the NbN, a consequent giant amplitude modulation is observed due to the strong nonlinearities. The high sensitivity of the chosen metamaterial even allows observing the nonlinear behaviors at various temperatures, but the resonance modulation induced by the nonlinear effect was distinct from that induced by the heating effect. The presented results illustrate a clever implementation of strongly enhanced nonlinearities and thus may bring nonlinear metamaterials into novel applications. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4794077]

DOI： 10.1063/1.4794077

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Other Link： http://orcid.org/0000-0003-0399-8104

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We measured reflectivity on static stripe-ordered La1.84-yNdySr0.16CuO4 (LNSCO) y = 0.1, 0.2, 0.3, and 0.4 by means of reflective terahertz time-domain spectroscopy (THz-TDS) with electric field polarized along c-axis, in which one can obtain lower frequency information than the conventional Fourier transform type spectrometer. Recently, two-dimensional superconducting (2DSC) property [1-3], intra-layer perfect conductivity not accompanied with inter-layer superconducting behavior, was reported on LNSCO (x = 0.15, y &gt;= 0.2). We observed the existence of Josephson plasma edge, which suggests that 2DSC is not realized in these compounds.

DOI： 10.1143/JPSJS.81SB.SB034

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We show that a charge-density wave mode submerged in large conductivity of the stripe-type charge-ordered (CO) crystal of Pr0.5Sr0.5MnO3 (PSMO) can be extracted by exciting selective crystallographic planes using polarized terahertz field. Low energy (1-7 meV) dynamics have been explored in the (100), (110), and (111) epitaxial films of PSMO manganite. Of (110) and (111) films representing bulk-like CO, the former exhibits a peak in the conductivity-energy spectrum with attributes of charge-density wave mode along a specific in-plane axis. On the contrary, the CO (111) and the phase-separated (100) films lack this low energy mode but exhibit a Drude-like spectral behavior in the metallic regime. These studies reveal a variety in isothermal charge dynamics originating from the anisotropic nature of the stripe charge-order. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772474]

DOI： 10.1063/1.4772474

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A simple approach to generate high energy, frequency and bandwidth tunable multicycle THz pulses by optical rectification (OR) of spatially shaped femtosecond laser pulses in the lithium niobate (LN) crystal is proposed and demonstrated. A one dimensional binary shadow mask is used as a laser beam shaper. By building the mask's image in the bulk LN crystal with various demagnifications, the frequency of THz generation was tuned in the range of 0.3 - 1.2 THz. There exist also an opportunity to tune the bandwidth of THz generation from 20 GHz to approximately 1 THz by changing the optical beam size on the crystal. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 0.18 mu J/THz for similar to 1 W pump power at 1 kHz repetition rate. (C) 2012 Optical Society of America

DOI： 10.1364/OE.20.025752

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

DOI： 10.1007/s10762-012-9928-4

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

We employed a laser terahertz (THz) emission microscope (LTEM) as a novel tool for evaluating solar cells. LTEM images are obtained by exciting a polycrystalline silicon solar cell with femtosecond laser illumination and visualizing the local distribution of the optical response. THz emission signals also provide various types of information, such as the screening effect of the built-in electrical field near pn junctions. These results indicate that this technique can be used to evaluate the local photoelectric conversion efficiency distribution and dynamic behavior of optically excited carriers in solar cells. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/apex.5.112301

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

A new scheme of optical rectification of femtosecond laser pulses in a periodically poled lithium niobate crystal, which generates high energy and linewidth tunable multicycle terahertz (THz) pulses, is analyzed. The developed simple theoretical model allows investigating the generated THz spectrum and its dependence on spot size of the pump beam. It is shown that the transformation of THz radiation from narrowband to broadband is possible by simply reducing the pump beam size. The temporal waveform and energy of the multicycle THz pulses were calculated as well. It is shown that THz energy is inversely proportional to the pump beam size r (y) , whereas energy spectral density is independent of r (y) . The efficiency of optical-to-THz energy conversion for pump pulse energy of 1 mJ is estimated to be 0.8 x 10(-4). The possibility of tuning the generation frequency by changing the direction of the pump beam propagation is analyzed.

DOI： 10.1007/s10762-012-9918-6

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

It is theoretically shown that application of a phase mask in optical rectification scheme is equivalent to spatial modulation of the crystal's nonlinear coefficient in cross-section plane of the laser beam. It allows using the technique of quasi-phase-matching for efficient noncollinear terahertz (THz) generation by using high-power wide-aperture optical beam. According to calculations, the linewidth of THz generation can be varied from 10 GHz to a few THz by changing the optical beam size. It is shown that the frequency of THz generation can be also tuned by building the image of the phase mask in the crystal with variable magnification. (C) 2012 Optical Society of America

DOI： 10.1364/OL.37.004155

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Authorship：Last author,　Corresponding author   Language：Japanese  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

The process of terahertz generation through optical rectification in a nonlinear crystal is modeled using discretized equivalent current sources. The equivalent terahertz sources are distributed in the active volume and computed based on a separately modeled near-infrared pump beam. This approach can be used to define an appropriate excitation for full-wave electromagnetic numerical simulations of the generated terahertz radiation. This enables predictive modeling of the near-field interactions of the terahertz beam with micro-structured samples, e. g. in a near-field time-resolved microscopy system. The distributed source model is described in detail, and an implementation in a particular full-wave simulation tool is presented. The numerical results are then validated through a series of measurements on square apertures. The general principle can be applied to other nonlinear processes with possible implementation in any full-wave numerical electromagnetic solver. (C) 2012 Optical Society of America

DOI： 10.1364/OE.20.018397

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Low-dimensional carbon nanostructures, such as single-wall carbon nanotubes (SWCNTs) and graphene, offer new opportunities for terahertz science and technology. Being zero-gap systems with a linear, photon-like energy dispersion, metallic SWCNTs and graphene exhibit a variety of extraordinary properties. Their DC and linear electrical properties have been extensively studied in the last decade, but their unusual finite-frequency, nonlinear, and/or non-equilibrium properties are largely unexplored, although they are predicted to be useful for new terahertz device applications. Terahertz dynamic conductivity measurements allow us to probe the dynamics of such photon-like electrons, or massless Dirac fermions. Here, we use terahertz time-domain spectroscopy and Fourier transform infrared spectroscopy to investigate terahertz conductivities of one-dimensional and two-dimensional electrons, respectively, in films of highly aligned SWCNTs and gated large-area graphene. In SWCNTs, we observe extremely anisotropic terahertz conductivities, promising for terahertz polarizer applications. In graphene, we demonstrate that terahertz and infrared properties sensitively change with the Fermi energy, which can be controlled by electrical gating and thermal annealing.

DOI： 10.1007/s10762-012-9916-8

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

We have proposed and developed a scanning laser terahertz (THz) near-field imaging system using a 1.56 mu m femtosecond fiber laser for high spatial resolution and high-speed measurement. To obtain the two-dimensional (2D) THz images of samples, the laser pulses are scanned over a 2D THz emitter plate [DASC: 4'-dimenthylamino-N-methyl-4-stilbazolium p-chlorobenzenesulfonate] by a galvano meter. In this system, THz wave pulses locally generated at the laser irradiation spots transmit through the sample set on the emitter, and the amplitude of the transmitted THz wave pulse is detected by using a typical THz time-domain spectroscopy (THz-TDS) technique. Using this system, we have succeeded in obtaining THz transmission images of a triangle shaped metal sheet of millimeter-size and a human hair sample with a spatial resolution of sub-wavelength order up to similar to 27 mu m (similar to lambda(THz)/28) at an imaging speed of about 47 seconds/image for 512 x 512 pixels. (C) 2012 Optical Society of America

DOI： 10.1364/OE.20.012959

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We have fabricated a centimeter-size single-layer graphene device with a gate electrode, which can modulate the transmission of terahertz and infrared waves. Using time-domain terahertz spectroscopy and Fourier-transform infrared spectroscopy in a wide frequency range (10-10 000 cm(-1)), we measured the dynamic conductivity change induced by electrical gating and thermal annealing. Both methods were able to effectively tune the Fermi energy, E-F, which in turn modified the Drude-like intraband absorption in the terahertz as well as the "2E(F) onset" for interband absorption in the mid-infrared. These results not only provide fundamental insight into the electromagnetic response of Dirac ferinions in graphene but also demonstrate the key functionalities of large-area graphene devices that are desired for components in terahertz and infrared optoelectronics.

DOI： 10.1021/nl301496r

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A new scheme of optical rectification (OR) of femtosecond laser pulses in a periodically poled lithium niobate (PPLN) crystal, which generates high energy and bandwidth tunable multicycle THz pulses, is proposed and demonstrated. We show that the number of the oscillation cycles of the THz electric field and therefore bandwidth of generated THz spectrum can easily and smoothly be tuned from a few tens of GHz to a few THz by changing the pump optical spot size on PPLN crystal. The minimal bandwidth is 17 GHz that is smallest ever of reported in scheme of THz generation by OR at room temperature. Similar to the case of Cherenkov-type OR in single-domain LiNbO3, the spectrum of THz generation extends from 0.1 THz to 3 THz when laser beam is focused to a size close to half-period of PPLN structure. The energy spectral density of narrowband THz generation is almost independent of the bandwidth and is typically 220 nJ/THz for similar to 1 W pump power at 1 kHz repetition rate. (C) 2012 Optical Society of America

DOI： 10.1364/OE.20.008784

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Optical Society of America  

DOI： 10.1364/OE.20.000042

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The ferroelectric soft mode in a SrTiO3 thin film was impulsively driven to a large amplitude using intense picosecond terahertz pulses. As the terahertz electric field increased, the soft-mode absorption peak exhibited blueshifting and spectral narrowing. A classical anharmonic oscillator model suggests that the induced displacement is comparable to that of the ferroelectric phase transition. The spectral narrowing indicates that the displacement exceeds that induced by any inhomogeneities in the film, demonstrating that the method can be used to explore intrinsic quartic anharmonicity.

DOI： 10.1103/PhysRevLett.108.097401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

In this letter, we report the characterization of the refractive indices and complex conductivities of a set of GaN films with different carrier concentrations, InN film, and InxGa1-xN films with indium content varying from x = 0.07 to x = 0.14 grown by metalorganic chemical vapor deposition for frequencies ranging from 0.3 to 3 THz using terahertz time-domain spectroscopy (THz-TDS). The refractive indices of InxGa1-xN films at THz range are reported. The carrier density and mobility determined using THz-TDS method show good agreement with four-probe Hall measurements. (C) 2012 American Institute of Physics. [doi:10.1063/1.3684836]

DOI： 10.1063/1.3684836

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We demonstrate a terahertz polarizer built with stacks of aligned single-walled carbon nanotubes (SWCNTs) exhibiting ideal broadband terahertz properties: 99.9% degree of polarization and extinction ratios of 10(-3) (or 30 dB) from similar to 0.4 to 2.2 THz. Compared to structurally tuned and fragile wire-grid systems, the performance in these polarizers is driven by the inherent anistropic absorption of SWCNTs that enables a physically robust structure. Supported by a scalable dry contact-transfer approach, these SWCNT-based polarizers are ideal for emerging terahertz applications.

DOI： 10.1021/nl203783q

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A SrTiO3 thin film was deposited on a MgAl2O4(100) substrate with a 3.5 % larger lattice constant than that of the SrTiO3. The temperature dependence of the dielectric dispersion of the SrTiO3 thin film was measured by terahertz time-domain spectroscopy, and the dielectric strength and soft-mode frequency showed positive and negative peaks at around 170 K, respectively. In addition, a deviation of the dielectric constant from the theoretical fitting curve in the gigahertz range and an increase in the value of the damping factor were observed below the ferroelectric transition temperature. These phenomena strongly indicate the emergence of ferroelectricity at around 170 K.

DOI： 10.1007/s10762-011-9839-9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Planar metamaterials consisting of subwavelength resonators have been recently proposed for thin dielectric film sensing in the terahertz frequency range. Although the thickness of the dielectric film can be very small compared with the wavelength, the required area of sensed material is still determined by the diffraction-limited spot size of the terahertz beam excitation. In this article, terahertz near-field sensing is utilized to reduce the spot size. By positioning the metamaterial sensing platform close to the sub-diffraction terahertz source, the number of excited resonators, and hence minimal film area, are significantly reduced. As an additional advantage, a reduction in the number of excited resonators decreases the inter-cell coupling strength, and consequently the resonance Q factor is remarkably increased. The experimental results show that the resonance Q factor is improved by more than a factor of two compared to the far-field measurement. Moreover, for a film with a thickness of lambda/375 the minimal area can be as small as 0.2 lambda x0.2 lambda. The success of this work provides a platform for future metamaterial-based sensors for biomolecular detection. (C) 2012 Optical Society of America

DOI： 10.1364/OE.20.003345

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

We demonstrate a tunable terahertz (THz) filter using an etalon with a nematic liquid crystal (NLC) layer. The etalon is composed of water-free fused silica glass plates and air layers, and contains a 75 mu m-thick defect layer filled with homeotropically-aligned NLCs. Frequency tuning of the transmission peak is achieved by applying an in-plane electric field across the NLC layer: the transmission peak shifts from 1.025 THz to 1.013 THz. The obtained rise and decay times are 5.3 s and 12.2 s respectively and found to be 10 times faster than previous THz. LC devices because of the thin NLC layer.

DOI： 10.1080/15421406.2012.686715

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.11906/TST.042-045.2011.06.05

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Terahertz (THz) radiation from InAs thin films grown by molecular-beam epitaxy on closely lattice-matched p-type GaSb (100) substrates and lattice-mismatched semi-insulating GaAs (100) substrates was investigated. The THz radiation intensity was measured from InAs films with thicknesses between 100 nm and 1.5 mu m excited by a femtosecond laser pulse with a wavelength of approximately 780 nm. The radiation intensity increased as the InAs film thickness increased and it exceeded that from a bulk n-type InAs substrate with an electron concentration of 2.3 x 10(16) cm(-3) when the InAs film thickness was greater than about 500 nm. In addition, the THz intensity from a 1-mu m-thick InAs film was greater than that from a bulk p-type InAs substrate. We ascribe this enhanced THz intensity to the wave reflected from the lower interface between the InAs film and the layer grown beneath it. We confirmed this by observing an increased pulse width due to constructive overlap of the reflected wave. The results demonstrate that InAs thin films are promising materials for THz emitting devices.

DOI： 10.1007/s10762-010-9694-0

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We have investigated the characteristics of THz emissions from p/n junctions with metallic lines under non-bias conditions. The waveforms, spectra, and polarizations depend on the length and shape of the lines. This indicates that the transient photocurrents from p/n junctions flow into the metallic lines that emit THz waves and act as an antenna. We have successfully demonstrated the non-contact inspection of open defects of multi-layered interconnects in a large-scale integrated circuit using the laser THz emission microscope (LTEM). The p/n junctions connected to the defective interconnects can be identified by comparing the LTEM images of normal and defective circuits. (C) 2011 Optical Society of America

DOI： 10.1364/OE.19.010864

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We have investigated the generation of THz radiation in lithium ternary compounds LiInSe(2), LiGaSe(2), LiInS(2), LiGaS(2) and characterized these materials by THz time-domain spectroscopy. Using 800 nm femtosecond excitation pulse, all crystals produce THz radiation due to an optical rectification corresponding to the nonlinear optical coefficient d(33). We have measured refractive indices along the x-axis and the z-axis for all crystals in the range 150-700 mu m and fitted them by using Sellmeier equation. With respect to the obtained results, velocity-matching between the incident laser pulse and the generated THz wave cannot be achieved at 800 nm, but for shorter wavelengths. Hence, an enhanced THz generation in Lithium ternary compounds may be observed by using a laser emitting below 800 nm.

DOI： 10.1007/s10762-011-9768-7

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY  

We have investigated terahertz (THz) emission from lithium ternary chalcopyrite crystals LiInSe(2), LiGaSe(2), LiInS(2), and LiGaS(2) that were illuminated by 1560 nm femtosecond pump laser pulses. Monocyclic THz emission caused by nonlinear optical effects was initially observed in all the illuminated crystals. Narrow-band THz emission from the coherent phonons were observed in LiInSe(2) (2.87 THz) and LiGaSe(2) (2.60 and 3.45 THz). These phonon modes were most likely caused by impulsive stimulated Raman scattering. Copyright (C) EPLA, 2010

DOI： 10.1209/0295-5075/91/20004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A terahertz (THz) chemical microscope (TCM) has been proposed and developed for label-free analysis of the interactions of proteins, including immune reactions. A SiO(2)/Si/sapphire plate was fabricated and emitted THz pulses from the sapphire side using a femtosecond laser. The amplitude of a THz pulse depends on the chemical potential of the SiO(2) surface. As an initial demonstration, a biotin-avidin complex was analyzed using TCM. Results indicate that biotin concentrations of up to 10(-12) mol/l could be detected, suggesting that a high sensitivity immunoassay system can be realized using this method. The distribution of the THz peak amplitude was measured, reflecting the distribution of the biotin-avidin complex on the plate. (C) 2010 American Institute of Physics. [doi:10.1063/1.3441408]

DOI： 10.1063/1.3441408

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Rubber compounds were investigated by terahertz time-domain spectroscopy. Terahertz absorption spectra of crude rubbers and additives were measured as well as those of acrylonitrile-butadiene rubber compounds, which included the additives. It was found that carbon black, which is one of the additives and serves as a filler, dominates the terahertz absorption owing to its metallic characteristics. Thus, terahertz spectroscopy is a useful method for rapid nondestructive inspection during the rubber production.

DOI： 10.1007/s10762-011-9832-3

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DOI： 10.11906/TST.026-032.2010.03.03

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Language：English   Publisher：ELSEVIER SCIENCE BV  

DOI： 10.1016/j.jlumin.2009.08.011

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

In this study we investigate anisotropic transmission properties of the membrane films impregnated with nematic LC materials in the THz frequency region. Typical experimental result shows that transmittance of membrane film without LC material is almost 100% in THz region. Phase change of the THz wave is observed but the transmittance does not decrease so much after impregnating LC material. Refractive index anisotropy increases after impregnating LC material which is related to the molecular orientation in the membrane film. These results show that the membrane film has a potential application to the LC device components in the THz region.

DOI： 10.1080/15421400903400894

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A high-sensitive scanning laser magneto-optical (MO) imaging system has been developed. The system is mainly composed of a laser source, galvano meters, and a high-sensitive differential optical-detector. Preliminary evaluation of system performance by using a Faraday indicator with a Faraday rotation coefficient of 3.47x10(-5) rad/mu m Oe shows a magnetic sensitivity of about 5 mu T, without any need for accumulation or averaging processing. Using the developed MO system we have succeeded in the fast and quantitative imaging of a rotationally symmetric magnetic field distribution around an YBa(2)Cu(3)O(7-delta) (YBCO) strip line applied with dc-biased current, and also succeeded in the detection of quantized fine signals corresponding to magnetic flux quantum generation in a superconducting loop of an YBCO Josephson vortex flow transistor. Thus, the developed system enables us not only to do fast imaging and local signal detection but also to directly evaluate both the strength and direction of a magnetic signal.

DOI： 10.1063/1.3276710

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Authorship：Lead author,　Last author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)  

Terahertz (THz) science and technology is now attracting increasing attention around the world, and materials, devices, and systems exploiting the THz waveband are set to play an important role in a very diverse range of applications. In this paper, we provide an overview of the current status and future prospects of the THz technology.

DOI： 10.3131/jvsj2.53.296

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We report a novel terahertz (THz) detector with a high-Tc superconducting (HTS) Josephson junction. This detector was fabricated using a YBa(2)Cu(3)O(7-delta) grain boundary junction. Clear THz signals induced by THz-pulse radiation from an ultrafast photoconductive antenna excited by femtosecond optical pulses were observed. We investigated the characteristics of the detector by changing the parameters of THz radiation such as pump laser power for the THz emitter and THz polarization. The results indicate that the detector mainly detects the THz electric fields. This is the first demonstration of a detection of pulsed THz electric fields using an HTS Josephson junction. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/APEX.3.042701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

The two stilbazolium derivatives, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) and 4-dimethylamino-N-methyl-4-stilbazolium p-chlorobenzenesulfonate (DASC), are organic ionic nonlinear optical materials that have a tendency to form bulk crystals when grown in a mixed solvent of methanol and acetonitrile. We observed the generation of broadband high-power terahertz (THz) waves from the bulk crystals of DASC. DASC crystals have superior transmission characteristics in the THz band than DAST crystals, and it is expected that the THz waves generated using DASC crystals will have higher power than those generated using DAST crystals. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.49.075502

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOCIETY APPLIED PHYSICS  

For the nondestructive inspection of gas hydrates, terahertz (THz) time-domain spectroscopy (TDS) was applied to tetrahydrofuran (THF) hydrate and propane hydrate. The absorption of propane hydrate monotonically increases with frequency, similar to the case of ice, while THF hydrate has a characteristic broad absorption peak at 0.5 THz corresponding to the dipole moment of THF molecules. The refractive indices of THF and propane hydrates are 1.725 and 1.775 at 1 THz, respectively, and show a slight but clear difference from the refractive index of ice (1.79). THz-TDS is a potentially useful technique for the nondestructive inspection of gas hydrates. (C) 2009 The Japan Society of Applied Physics DOI: 10.1143/APEX.2.122303

DOI： 10.1143/APEX.2.122303

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The properties of an YBa(2)Cu(3)O(7-d) superconducting quantum interference device (SQUID) have been evaluated magneto-optically and electrically. It was found that the operating characteristics of the SQUID observed by means of the magneto-optical (MO) method show a good consistency with those measured by the electrical method Furthermore, we have succeeded in the MO detection of local magnetic signals corresponding to the single flux quantum pulses generated in the SQUID loop applied with an external pulsed magnetic field. These results indicate that the MO probe has the potential to be applied to an optical output interface for superconducting magnetic flux devices. (C) 2009 American Institute of Physics. [doi:10.1063/1.3262957]

DOI： 10.1063/1.3262957

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Other Link： http://orcid.org/0000-0003-0399-8104

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

We measured strain effects of SrTiO3 (STO) thin films by terahertz time-domain spectroscopy (THz-TDS) with a 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal that was expected to be a broadband THz emitter. STO thin films were deposited on MgAl2O4, MgO, and LSAT substrates, and lattice constants of the films changed from the value of unstrained STO bulk because of lattice mismatches between STO and the substrates. The dielectric dispersions of the STO thin films deposited on MgO and MgAl2O4 were measured up to 6.0 THz by the broadband THz-TDS system and all the features of dielectric peaks of soft modes responsible for ferroelectricity were clearly observed. The soft-mode frequencies of the films shifted from the original value of STO bulk owing to the strain effects. In particular, the softening of the soft mode of the film on MgAl2O4, which implied that the in-plane tensile strain enhanced ferroelectric fluctuations, was of interest. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.09KA16

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

We have proposed and developed a novel technique for a non-contact inspection of defective interconnections in an LSI chip using a laser terahertz emission microscope (LTEM). The LTEM measures the THz emission images of an LSI chip by scanning it with fs laser pulses. When a fs laser pulse irradiates a p-n junction in an LSI chip, transient photocurrent flows into interconnections resulting in the emission of the THz pulse into free space. We investigated the characteristics of the THz emissions from simple test element group samples which consist of p-n junctions connected to metal lines. It was found that the metallic lines connected to photo-excited p-n junctions worked as THz emission antennae which enhance the emission efficiency of THz pulses near their resonant frequencies corresponding to the line lengths. This result indicates that THz emission signals from p-n junctions in circuits strongly depend on the structure of the interconnections. We show the successful results on the inspection of defective interconnections in MOSFET devices and C7552 ISCAS&apos;85 benchmark circuits using LTEM. By comparing the THz emission images between a normal circuit and a defective one, it is possible to identify the p-n junctions connected to the defective interconnections without electrical contacts. (C) 2009 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.microrel.2009.07.047

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

In this paper, the angular dependence of terahertz (THz) emission from semiconductor surfaces photoexcited by femtosecond optical pulses is reported. Time-domain waveforms of THz emission from ( 100) surfaces of semi-insulating gallium arsenide (si-GaAs) and p-type indium arsenide (p-InAs) are measured at various angles after careful suppression of the nonlinear optical rectification effect. THz emission angle-frequency patterns under focusing conditions of the excitation beam are regarded as radiation from an electric dipole moment located on the semiconductor surface. Based on the experimental results in the magnetic field parallel to the semiconductor surface, we discuss the ultrafast carrier dynamics on the surfaces of both semiconductors. (C) 2009 Optical Society of America

DOI： 10.1364/JOSAB.26.000A14

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

Terahertz (THz) time-domain spectroscopy is used as a noncontact method to evaluate a nondoped indium phosphide (InP) wafer for the temperature and frequency ranges of 4.2-300 K and 0.2-4 THz, respectively. The strongly temperature- and frequency-dependent optical constants of the complex refractive index and complex conductivity were observed in the THz region, which were fitted and analyzed with a simple Drude model. The temperature dependence of the carrier density and scattering time are also presented. The shallow donors of impurities are discussed with the obtained results. (C) 2009 Optical Society of America

DOI： 10.1364/JOSAB.26.0000A1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have developed a terahertz time-domain spectroscopy (THz-TDS) system for reflectivity measurement with a temperature-controllable cryostat. For emission and detection of THz radiation. a Ti:Sapphire pulsed laser and photoconductive antennas are used. Two wire-grid polarizers enable us to carry Out the polarized reflectivity measurements. Using Our THz-TDS system, we measured the c-axis polarized reflectivity spectra for La2-xSrxCuO4 Single crystals (x = 0.10 and 0.13) and observed sharp Josephson plasma edges for both samples below T-c. The reflectivity spectra of the x = 0.10 sample were in good agreement with those reported previously, which confirms the validity of the system. For the x = 0.13 sample, we discuss the anomolous features of the optical spectra. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.05.188

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

The crystallographic anisotropy in electromagnetic terahertz radiation from multiferroic BiFeO3 (100), (110), and (111) films suggests the ultrafast depolarization of ferroelectric order as a new mechanism of terahertz emission. This implies that the spontaneous polarization in ferroelectric materials can be manipulated in time scale of picoseconds, thus paving the way to ultrafast data-storage devices based on nonvolatile ferroelectric memories.

DOI： 10.1002/adma.200802094

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

We describe a film of highly aligned single-walled carbon nanotubes that acts as an excellent terahertz linear polarizer. There is virtually no attenuation (strong absorption) when the terahertz polarization is perpendicular (parallel) to the nanotube axis. From the data, the reduced linear dichrosim was calculated to be 3, corresponding to a nematic order parameter of 1, which demonstrates nearly perfect alignment as well as intrinsically anisotropic terahertz response of single-walled carbon nanotubes in the film.

DOI： 10.1021/nl900815s

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have fabricated a superconducting vortex flow transistor (SVFT) device with a YBa(2)Cu(3)O(7-delta) (YBCO) thin film on an MgO substrate. To detect the local magnetic flux state inside the superconducting loop of the device, we utilized a scanning laser magneto-optical microscope (SLMO). As a result we have succeeded in the observation of magnetic flux distribution in the device by applying dc-control current larger than 100 mA. Moreover, we could detect the time variation of local magnetic field in the superconducting loop as the MO signals by applying ac-control current with an amplitude of 100 mA at 500 Hz.

DOI： 10.1109/TASC.2009.2018536

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Language：Japanese  

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Santa Barbara, USA 7-11 March, 2009

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Bis[4-dimethylamino-N-methyl-4-stilbazolium] terephthanate (BDAS-TP) was newly designed as an organic nonlinear optical crystal. It is derivative of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), which is well known as a novel terahertz (THz)-wave emitter. We also succeeded in growing single crystals of BDAS-TP for the first time by using a solution-slow-cooling technique. By using a femtosecond laser, we obtained a broadband THz spectrum from BDAS-TP crystal. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2008.09.080

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A terahertz chemical microscope (TCM) has been developed to visualize the chemical concentration of solutions in fluidic chips. Chemical potential-terahertz transducers were newly developed and applied to the bottom surfaces of the flow channels. When femtosecond laser pulses hit the transducers, terahertz waves are radiated from them. Since the amplitude of terahertz waves can be related to the chemical potential on the surface of the device, the distribution of chemical reactions on the surface can be visualized by scanning the laser on the device. Here, we report the demonstration of mapping of pH values using TCM with a spatial resolution of less than 1 mm. The pH distribution of the flow channels filled with 0.01 M NaOH and HC1 was successfully visualized. In addition, the neutralization process was monitored. © 2009 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejsmas.129.221

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institution of Engineering and Technology (IET)  

DOI： 10.1049/el:20030636

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A new type of terahertz (THz) radiation imaging has been proposed and demonstrated. The supercurrent distributions were visualized by detecting the backscattered THZ radiation from optically excited high-temperature superconductors. The supercurrent distributions were clearly observed in 100- and 500-nm-thick YBa2Cu3O7-delta films in the remanent state. Our system has performance similar to the system With transmission configuration.

DOI： 10.1109/TASC.2003.812428

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Other Link： http://orcid.org/0000-0003-0399-8104

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Reversible and bistable terahertz (THz) radiation by sweeping the temperature has been observed in a dipole-type photoswitching device fabricated on magnetoresistive charge-ordered manganite Pr0.7Ca0.3MnO3 thin film. Based on simultaneous measurements of the photocurrent, we show that this THz functionality of Pr0.7Ca0.3MnO3 arises from the instability of metallic patches embedded with a charge-ordered insulating phase, which is created by the illumination of visible light under an electric field at constant temperature, and that it can also be controlled by a subsequent change of temperature. (C) 2003 American Institute of Physics.

DOI： 10.1063/1.1575507

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

THz time-domain spectroscopy was used to directly probe the low-energy (0.5-5 meV) electrodynamics of the charge-ordered manganite Pr0.7Ca0.3MnO3. We revealed the existence of a finite peak structure around 2-3 meV well below the charge gap ∼300 meV. In analogy to the low-energy optical properties of the well-studied low-dimensional materials, we attributed this observed structure to the collective excitation mode arising from the charge-density-wave condensate. This finding provides the importance role of the quasi-one-dimensional nature of the charge and orbital ordering in Pro0.7Ca0.3MnO3.

DOI： 10.1103/PhysRevB.66.024401

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have observed the free-space radiation of the resonant terahertz wave from c-axis oriented Tl2Ba2CaCu2O8+delta thin films by femtosecond optical pulse excitation under a radial magnetic field of about 100 Oe nearly parallel to the c axis of the thin film. The observed wave form showed clear oscillations up to 80 K. The frequency was shifted from similar to620 GHz at 24 K to similar to300 GHz at 80 K, corresponding to an increase in c-axis penetration depth, lambda(c), from similar to26 mum to similar to53 mum, and disappeared above T-C as expected for the Josephson plasma resonance. (C) 2002 American Institute of Physics.

DOI： 10.1063/1.1475770

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We optimized the aspect ratio of a sapphire puck for a Hakki-Coleman-type high-temperature superconductive (HTS) thin film shielded sapphire dielectric resonator (SDR) to get the maximum Q-value, Q(0), of the SDR at the frequencies 10, 15 and 20 GHz. For the simulation, we used the exact expressions for the geometrical factors of various parts of the SDR obtained by the Q-value definition. The SDR analysis is made by using HTS thin films deposited on 1 inch diameter and 10 mm x 10 mm substrates. The analysis suggests that the optimum aspect ratio of the sapphire puck depends on the surface resistance of the HTS thin film to get the maximum Q-value of the SDR. This analysis can be very useful for the design of a HTS shielded sapphire SDR of desired size within the available space for its application.

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Femtosecond optical pulses were irradiated into a YBa2Cu3O7-δ strip-line of 400 μm in width in order to investigate the relationship between the laser beam power profile and the distribution of the optically generated magnetic fluxes. To homogenize the current distribution in the strip-line, a half of the strip-line was patterned into the ordered array structure of holes of 2 μm in diameter at an interval of 4 μm, and several experimental conditions were examined by changing the focal size of the pulsed laser beam at the sample surface. As a result, it was found out that the generated optical magnetic fluxes strongly depend on the power profile of the femtosecond optical pulses, and showed a possibility for the application to a laser beam profiler and the other optical devices.

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Terahertz (THz) radiation with its spectrum extending up to 1 THz has been observed by an illumination of femtosecond optical pulses to optical switching devices fabricated on magnetoresistive manganite thin films; Pr0.7Ca0.3MnO3. The THz radiation strongly depends on temperature T and its T trend reverses sign across charge-orbital and spin-ordering T. (C) 2001 American Institute of Physics.

DOI： 10.1063/1.1381573

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Direct experimental estimations of the low-energy (1.5-10 meV) complex dielectric constants spectrum and its temperature variation have been investigated for La0.7Ca0.3MnO3 thin films using terahertz time-domain spectroscopy. At low temperatures, a clear Drude term emerges. With increasing temperature, the scattering rate increases, while the plasma frequency decreases, both derived from a simple Drude model. Finally, a Drude term submerges well below the insulator-metal transition temperature. On the basis of the present results, low-energy charge dynamics are discussed.

DOI： 10.1103/PhysRevB.62.R11965

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：INST PURE APPLIED PHYSICS  

Femtosecond optical response in GaAs thin films has been studied, We prepared GaAs thin films on MgO substrates and on YBa2Cu3O7-delta (YBCO) thin films using pulsed laser deposition (PLD) at temperatures below 250 degreesC, A photocarrier lifetime of less than 1 ps is measured for the prepared GaAs thin films using femtosecond time-domain reflectivity change measurements. Pulsed electromagnetic wave [terahertz (THz) radiaiton] containing a frequency component of up to 1 THz is emitted from fabricated photoconductive switches using the prepared thin films. We also evaluated the THz radiation properties emitted from the photoswitches on the YBCO thin films.

DOI： 10.1143/JJAP.39.6304

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Language：English   Publishing type：Research paper (scientific journal)  

Direct experimental estimations of the low-energy (1.5-10 meV) complex dielectric constants spectrum and its temperature variation have been investigated for La0.7Ca0.3MnO3 thin films using terahertz time-domain spectroscopy. At low temperatures, a clear Drude term emerges. With increasing temperature, the scattering rate increases, while the plasma frequency decreases, both derived from a simple Drude model. Finally, a Drude term submerges well below the insulator-metal transition temperature. On the basis of the present results, low-energy charge dynamics are discussed.

DOI： 10.1103/PhysRevB.62.R11965

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We studied the optical creation and modulation of magnetic flux quanta in a YBa2Cu3O7-delta thin film loop by observing terahertz emission excited with femtosecond laser pulses. The magnetic flux is created and modulated in the loops with a polarity controlled by means of optical supercurrent modulation. The results suggest a possible application of the superconductive loop as an optical flux-trap memory for the detection and storage of the femtosecond optical signals. The mechanism for the creation of the magnetic flux is discussed in relation to the current distribution in the loop.

DOI： 10.1109/77.784017

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

We have demonstrated that the supercurrent distribution in current-biased YBa2Cu3O7-δ thin films can be obtained by measuring the radiation power of THz electromagnetic pulses excited with femtosecond laser pulses. As the radiation power is proportional to the square of the bias current density at the laser spot position, the two-dimensional current distribution can be obtained from the intensity distribution of THz radiation by scanning the laser spot. The characteristic supercurrent distribution is analyzed by using the critical-state model. © 1999 American Institute of Physics.

DOI： 10.1063/1.123536

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

Magnetic flux generated optically in YBa2Cu3O7-delta thin film loops was visualized by means of terahertz (THz) radiation imaging. By scanning the laser spot on the sample surface, we obtained a two-dimensional radiation pattern that represents supercurrent distribution. The observed images revealed that the fs laser pulse can create magnetic flux in the superconductive loops and also control the polarity of the flux. We discuss the relationship between the supercurrent distribution and the optically generated magnetic flux in the loop.

DOI： 10.1143/jjap.37.L1301

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

A superconducting optical flux-trap memory with a femtosecond pulsed laser is proposed and then demonstrated. The magnetic flux is trapped in a simple YBa2Cu3O7-δ thin film loop with a polarity controlled by means of optical supercurrent modulation. After removing the external bias current, the trapped fluxes are detected by observing electromagnetic pulse radiation 60 cm away from the sample, which is excited by modulating a persistent current that circulates in the loop. The mechanism for the readin of the magnetic fluxes is discussed in relation to the current distribution in the memory. © 1997 American Institute of Physics.

DOI： 10.1063/1.120029

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Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.3390/condmat10010001

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DOI： 10.3390/photonics10091024

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

We have proposed highly efficient microcavity second harmonic generation devices. In this work, we designed and fabricated an a-plane GaN vertical monolithic microcavity second harmonic generation device pumped with a femtosecond laser and obtained normalized wavelength conversion efficiency of 0.15% W⁻¹. The efficiency was comparable to the theoretical estimations taking into account the time evolution of fundamental wave intensity in the microcavity, second harmonic generation and even sum frequency generation. This result indicates the possibility of realizing ultra-compact and ultra-efficient devices pumped with long-pulsed or continuous wave lasers, in which the effect of resonance enhancement is even more pronounced.

DOI： 10.35848/1882-0786/ace242

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ace242/pdf

Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Far-UV light sources have attracted much attention for human-safe viral inactivation and bacterial disinfection. Due to large optical nonlinearity and transparency to this wavelength region, AlN is a promising material for compact and low-cost far-UV second harmonic generation (SHG) devices. In this study, a transverse quasi-phase-matched AlN channel waveguide with vertical polarity inversion was designed and fabricated. From wavelength spectra and a pump power dependence of an SH intensity, far-UV SHG via the largest nonlinear optical tensor component d₃₃ was successfully confirmed under ultrashort pulse laser excitation.

DOI： 10.35848/1882-0786/acda79

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/acda79/pdf

Publishing type：Research paper (scientific journal)   Publisher：Optica Publishing Group  

DOI： 10.1364/AO.487770

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Authorship：Last author   Language：English  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

A wavelength conversion device with an input grating coupler that enables easy alignment was fabricated using a vertically polarity inverted GaN layer. The device was excited with a femtosecond laser, and a second harmonic wave with a peak wavelength of 438.4 nm was obtained. A normalized wavelength conversion efficiency of 4.7  10−4%W−1 and a spectral bandwidth of the second harmonic wave of 2.9 nm were comparable to the theoretical estimations taking into account serious walk-off and waveguide losses. High efficiency of 33-34%W−1 can be expected under continuous-wave excitation.

DOI： 10.35848/1882-0786/ac9511

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/ac9511/pdf

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

In this study, we focus on the terahertz radiation from a Si metal-oxide-semiconductor (MOS) structure and formulate the mechanism of THz radiation to semi-quantitatively evaluate the MOS devices. A simplified model, which can effectively explain the dependence of THz emission amplitudes on external bias voltages for both n- and p-type Si MOS structures, is derived from Poisson's equation with considering external effects, including the flat-band voltage, hot carrier diffusion, and quick recombination through interface states due to photocarrier field acceleration. The results reveal the possibility on estimating the surface parameters, such as surface potential and dielectric layer.

DOI： 10.1109/IRMMW-THz50927.2022.9896119

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We present a compact terahertz (THz) microfluidic chip based on a nonlinear optical crystal with a small number of meta-atoms for accelerating the research on THz microfluidics. The chip operates based on near-field interactions among a solution sample that flows in a microchannel, meta-atoms, and a highly dense point THz source that occurs in the process of optical rectification at the irradiation spots of femtosecond pulse laser beam. To date, we have achieved attomole-level sensing using a picoliter-order volume of various solutions in a sub-wavelength effective sensing area. Details of the chip, sensing examples, and future THz microfluidics are discussed.

DOI： 10.1109/IRMMW-THz50927.2022.9895891

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We report the terahertz (THz) emission spectroscopy (TES) and photoluminescence (PL) spectroscopy results for a semi-insulating (SI) GaN film in comparison with those for unintentionally-doped (UID) and magnesium (Mg)-doped ones. The THz emission polarity showed their band bending near the surface. In addition, the TES and PL results showed intensity changings over time for SI, UID-GaN, but not for Mg-doped GaN. The origin of these effects was studied by observing the THz emission property in the air and vacuum and attributed to the UV- induced hydrogen dissociation.

DOI： 10.1109/IRMMW-THz50927.2022.9895543

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

The primary cause of the THz radiation from semiconductors is the optically created oscillating dipoles and their orientations. Therefore, the optical excitation angles of the incident femtosecond laser and THz detection vary due to the radiation patterns of dipoles considering Snell's law and the phased-array effect. We investigated this phased-array effect using finite-difference time-domain simulations for angular dependence at focus and non-focus conditions of the laser beam spot. The angular dependence simulation results at focus conditions show a point-source dipole radiation pattern and a pronounced directivity for high frequencies at non- focus conditions. This study provides a better understanding of the THz radiation pattern from semiconductors.

DOI： 10.1109/IRMMW-THz50927.2022.9895821

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

We proposed a monolithic microcavity wavelength conversion device without a polarity-inverted structure. The device consists of a low birefringence paraelectric material and a dielectric material. A fundamental wave intensity is enhanced significantly in the microcavity with two distributed Bragg reflectors, and a second-harmonic wave is efficiently generated in a very short region close to a coherence length. As a first step of this study, we used GaN as a device material, and succeeded in the blue second harmonic generation with a wavelength of 428 nm.

DOI： 10.35848/1882-0786/abff9e

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Other Link： https://iopscience.iop.org/article/10.35848/1882-0786/abff9e/pdf

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers (IEEE)  

Magnetite (Fe3O4) nanoparticles (NPs) of diameter 100 nm and nano-hollow spheres (NHSs) of diameter 100, 185, 250, 350, and 725 nm have been synthesized by a facile one-step template-free solvothermal technique. Silica (SiO2) coating on their surface has been carried out following the Stober method. X-ray diffraction and high-resolution transmission electron microscope images confirmed the phase and morphology of the nanostructures (NSs). Furthermore, we have investigated the terahertz (THz) wave absorption properties of SiO2@Fe3O4 shell/shell NSs in the frequency range of 0.9-2.0 THz. Detailed morphology and size-dependent THz absorption study proves that these particles can be very useful as electromagnetic shielding devices.

DOI： 10.1109/tmag.2021.3056575

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Gallium oxide are a potential material for many applications and recent research showed its strong anisotropic properties in THz range. We applied THz-TDS and CW laser to study the excited carrier dynamics in our sample (Ga2O3, 010, Fe-doped). The results showed different response along two crystal axes. Along the a-axis we observed Drude response and conductivity are increasing with CW laser increasing, as the same as we expected. Our fitting results (by Drude model) of this direction also agreed with our expectation. Along the c-axis we observed non-Drude response and further experiments are needed.

DOI： 10.1109/IRMMW-THz50926.2021.9567220

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A laser terahertz emission microscope (LTEM) will be a killer tool as a real-world application. Here various examples are reviewed to prove it.

DOI： 10.1109/IRMMW-THz50926.2021.9567150

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We performed 3D imaging of a clothed human using a single millimeter-wave MIMO radar. By performing clustering and smoothing processes based on the observed intermediate frequency signals, 3D image reconstruction was successfully achieved.

DOI： 10.1109/IRMMW-THz50926.2021.9567037

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We studied the evolution of the two-color laser filament emitted THz pulse energy with the influences of the infrared pump energy and air dispersion. The THz modulation induced by additional relative phase is verified by the two-color spectral broadening due to the intensity dependent nonlinearity.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE Computer Society  

Recent progress on the development of a scanning point terahertz source (SPoTS) and its application to biosensing and cancer imaging is reported.

DOI： 10.1109/IRMMW-THz46771.2020.9370823

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

A compact terahertz microfluidic chip based on a Fano resonance of meta-atoms was demonstrated for the ultra-trace measurements of solutions. We successfully detected attomole order of solutes in a 77 pL volume of the solution.

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DOI： 10.1007/s10762-020-00687-1

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

A compact terahertz microfluidic chip based on a Fano resonance of meta-atoms was demonstrated for the ultra-trace measurements of solutions. We successfully detected attomole order of solutes in a 77 pL volume of the solution. (C) 2020 The Author(s)

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We have developed an apertureless terahertz (THz) near-field microscope and a nonlinear optical crystal-based THz microfluidic chip for highly sensitive and high spatial resolution THz analysis of biological samples. Using these systems, we have succeeded in observing a clear THz image of a breast cancer tissue with a resolution of micrometer scale and in detecting solutes of attomole scale in a less than 100 pL volume of the solution.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We have measured terahertz emissions form (010) and ( overline{2} 01) surfaces of beta-Ga O that is recently attracting significant attention as a candidate for next generation power electronics. The surfaces of Ga O were systematically investigated using laser THz emission microscopy and the results showed clear dependences of THz waveforms on carrier concentrations and surface orientations. 2 3 2 3

DOI： 10.1109/irmmw-thz.2019.8874422

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© 2018 IEEE. We developed a PDMS microchannel integrated type terahertz chip for trace analysis of liquid solution. By utilizing near-field interactions of locally generated THz waves with a few arrays of metamaterials and the liquid solution flowing inside the PDMS microchannel, we found the possibility of high-sensitive and quantitative sensing of the trace amount of liquid solution with different concentration.

DOI： 10.1109/IRMMW-THz.2018.8510260

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE Computer Society  

Hot electron injection utilizing a GaSb/lnAs heterostructure was studied in order to enhance terahertz radiation from a semiconductor surface. A 1-nm-thick InAs surface layer served to alleviate the large surface band bending due to Fermi level pinning at the GaSb surface. As a result, a stronger THz radiation was obtained as compared to that from an InAs thin film. This demonstrates that the use of InAs/GaSb/lnAs heterostructures is effective in enhancing electron diffusion, and hence, THz radiation.

DOI： 10.1109/IRMMW-THz.2018.8510477

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We have previously reported how semiconducting and amorphous Y-Ba-Cu-O (alpha-YBCO) can perform fast and sensitive infrared pyroelectric detection. We recall first these performances for planar and trilayer alpha-YBCO structures. We then consider migration to THz sensing by using a planar antenna. To minimize the antenna to device impedance mismatch, alpha-YBCO THz conductivity was measured. The conductivity, obtained by parallel plate waveguide time-domain spectroscopy, is 50 to 150 S/m (0.2 to 1.2 THz range). Antenna structures are suggested for both planar and trilayer devices.

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Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OSA - The Optical Society  

We developed a reflection type scanning laser THz near-field spectroscopy and imaging system and evaluated its basic performance. We found that this system has huge potential for high-resolution, high-sensitive and high-speed biological measurements.

DOI： 10.1364/CLEO_SI.2018.SW3D.4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical and Electronics Engineers Inc.  

The papers in this special section highlight the recent progress and trends in the research of terahertz photonics.

DOI： 10.1109/JSTQE.2017.2722846

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SURFACE SCI SOC JAPAN  

Physically adsorbed gas molecules have dramatic effects on the inherent properties of 2D nanomaterials due to their atomic/molecular-level thinness In this work, we applied laser THz emission spectroscopy to study the gas adsorption-desorption dynamics of tungsten disulfide (WS2) nanosheets prepared by liquid phase exfoliation technique. We find that in low vacuum conditions, NIR irradiation promotes desorption of 02 molecules from WS2 nanosheets. Upon exposure to atmospheric air, results suggest slight photo-oxidation caused by NIR irradiation. Strong photo-oxidation by UV illumination is also confirmed, and we also observed subsequent 02 desorption from WS2 nanosheets by NIR irradiation when the UV illumination is turned off. These results are discussed using the Langmuir adsorption isotherm, which explains the variation of gas adsorption with the gas pressure.

DOI： 10.1380/ejssnt.2015.78

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

The newly discovered atomically thin and layered materials which host electronic system that respond to long wavelength light in extraordinary manner can lead to a major breakthrough in the field of terahertz (THz) optics and photonics. However, their low conductivities due to either low densities or low mobility make it challenging to characterize their basic THz properties with the standard spectroscopic method. Here, we develop a THz spectroscopic technique based on parallel plate waveguide (PPWG) to overcome the limitations of the conventional THz time domain spectroscopy (TDS) technique. The present method is particularly suitable to ultrathin conductive materials with low carrier density. We report in details the derivation of the dispersion equations of the terahertz wave propagation in a PPWG loaded by a thin conductive materials with zero -thickness. These dispersion equations for transverse magnetic (TM) and transverse electric (TE) waveguide modes are the core of the optical parameters extraction algorithm in the THz-PPWG-TDS analysis. We demonstrate the effectiveness of the waveguide approach by characterizing low conductive CVD graphene. The high sensitivity of THz-PPWG-TDS technique enables us to study the carrier dynamics in graphene with Drude and Drude-Smith model.

DOI： 10.1117/12.2225060

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SCIENCEDOMAIN International  

DOI： 10.9734/PSIJ/2015/15338

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Recently, metamaterial in the terahertz (THz) region has developed as a novel approach to control and manipulate the electromagnetic waves. In this paper, we demonstrated our recent development of nonlinear THz metamaterial induced by intense THz field. The ultrafast dynamics of the nonlinear process is also observed by THz pump THz probe system.

DOI： 10.1109/APCAP.2015.7374419

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Other Link： http://orcid.org/0000-0003-0399-8104

Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPIE  

DOI： 10.1117/1.OE.53.3.031201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

DOI： 10.1088/0957-4484/24/21/210201

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DOI： 10.4236/ampc.2013.31009

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Other Link： http://www.scirp.org/journal/doi.aspx?DOI=10.4236/ampc.2013.31009

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Recently, considerable studies have shown that plasmon induced transparency (PIT) effect can be realized by metamaterials via destructive interference between different resonance modes. Here we present a few recent developments of PIT research in terahertz metamaterials.

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A heavily-doped InAs layer was grown as a reflection layer for enhancing THz radiation. Si δ-doped and uniformly-doped structures were studied. The electron mobility of Si δ-doped InAs was higher than that of uniformly-doped one. The intensity from samples with the reflection layer was stronger than that from that without the reflection layer. This shows that THz radiation intensity from InAs thin films can be enhanced by the reflection layer. © 2012 IEEE.

DOI： 10.1109/IMFEDK.2012.6218609

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：TAYLOR & FRANCIS LTD  

Twisted nematic type THz polarization control devices using liquid crystal immersed membrane films were fabricated by using simple stacking process. First of all, we verify the liquid crystal alignment effect in various kinds of membrane films. From THz Time-Domain Spectroscopy measurements, apparent refractive index anisotropy was observed in liquid crystal immersed Polytetrafluoroethylene and Polyolefin membrane films. Furthermore, transmission properties of twisted nematic type THz device are in rough agreement with 4x4 matrix calculation data. We believe that primary characteristic of THz TN device can be obtained by optimizing the fabrication processes and measurement method.

DOI： 10.1080/15421406.2011.568334

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DOI： 10.1007/s10762-011-9791-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

DOI： 10.1007/s10762-011-9791-8

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We examine InP and InGaAs Schottky type photoconductive antenna for the THz generation excited by femtosecond laser(fs) at a wavelength of 1560nm. Since InP has an energy gap of 1.3 eV, which is much larger than photon energy of fs laser. We obtained THz wave generation from both PC antennas with sufficient THz amplitude, which is comparative to that of low temperature grown GaAs. We also developed new type of InGaAs PC antennas which includes insulating gap in a PC structure. Highly bright THz beam was generated from the InGaAs PC antennas.

DOI： 10.1117/12.887938

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We have developed the laser terahertz emission microscope that does not require Vdd-biasing and signal input: NB (non-bias)-LTEM. The NB-LTEM, also, does not require signal output via electrodes. In this paper, first of all, the position of non-electrical contact tools, in failure analysis, is show. Secondary, the basic concept and the system schematic of the NB-LTEM are described. Thirdly, how to use the NB-LTEM in fault localization is explained. Then, some examples of fault localization or defect detection using the NB-LTEM are shown. Finally, we would like to compare the NB-LTEM and the other non-electrical contact tool (L-SQ, laser SQUID microscope).

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Language：English   Publishing type：Research paper (international conference proceedings)  

Transmission-type terahertz time-domain spectroscopy (THz-TDS) using dipole-antennas on LT-GaAs layers as THzemitter and detector with fs-fiber laser as the excitation light source was applied to determine the stereoisomer of sugars in carefully controlled environment with low humidity below 5% at 20°C. Commercially available α-D-lactose including about 4% anomer and β-D-lactose including below 30% anomer were used as stereoisomer samples. The absorption spectra of compounds by the α-D- and β-D-lactose powders with various ratios were successively deconvoluted to four spectra by Lorentzian and dependent on the composition ratio. Integrated absorptions due to α-D-lactose and β-D-lactose determined the decreasing rate (r α) and the increasing rate (r β) for the composition ratio of the β-D-lactose powder, because the intensities were linearly dependent on the composition ratio. The net-composition ratio of α-lactose and β-lactose in the compounds was precisely evaluated by the ratio of the integrated intensities and the value of r β / r α, for example, the anomer-contents in commercially available α-D-lactose and β-D-lactose powders were disclosed as 3.9% and 29.1%, respectively. The demonstrated results indicate that THz-TDS is so useful not only for precise qualitativeanalysis but also for precise quantitative-analysis of stereoisomer in sugars with partially different molecular structure such as lactose. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

DOI： 10.1117/12.902044

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

Recent progress of laser terahertz (THz) emission microscope (LTEM) is reviewed.Femtosecond lasers can excite the THz waves in various electronic materials due to ultrafast current modulation. The current modulation is realized by acceleration or deceleration of photo-excited carriers, and thus LTEM visualizes dynamic photoresponse of substances. The developed systems have a minimum spatial resolution better than 1.0 μm, which is defined by the laser beam diameter. We alos provide esamples of LTEM application to material science and semiconductor devise failure. © 2010 SPIE.

DOI： 10.1117/12.887937

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Language：Japanese  

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Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Anharmonicity in the soft-mode potential of quantum-paraelectric SrTiO3 thin films has been directly observed with strong terahertz wave generated from a LiNbO3 single crystal. Hardening of the soft-mode resonance has been detected. (C) 2009 Optical Society of America

DOI： 10.1364/CLEO.2010.CTuBB1

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Language：Japanese   Publishing type：Research paper (other academic)  

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Authorship：Lead author,　Corresponding author  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

A high-speed and a high-sensitive laser scanning magneto-optical (MO) imaging system have been developed. In the high-speed imaging mode, we have succeeded in almost the real time observation of MO images with the sensitivity better than 100 mu T. On the other hand, in the high-sensitive mode using an acoustic-optic modulator and a RF lock-in amplifier, it was found that the system noise reduced into the magnetic field strength was less than 6 mu T at the data acquisition time of 10 mu s. In this mode we have also succeeded in the fast MO imaging of the magnetic flux penetration into a square-shaped YBa(2)Cu(3)O(7-delta) n film at similar to 0.3 s/frame for 128x128 and similar to 5 s/frame for 500x500 pixels&apos; image.

DOI： 10.1088/1742-6596/150/1/012029

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Terahertz (THz) Technology is of great interest for many applications, and THz electronics is a key field to lay the base. Superconductive devices are promising ones as THz detectors, THz frequency signal processors and so on. Optical functionality of the devices is essential for the high speed devices and applications. In this review, we will introduce our recent activities to establish "Superconductor Photonics" and discuss its ultrafast optical functionalities with Josephson junctions and vortex flow transistors by means of pump and probe optical responses and a dynamic magneto-optical microscope.

DOI： 10.1109/TIC-STH.2009.5444378

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Other Link： http://orcid.org/0000-0003-0399-8104

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

To develop the hydrogen sensor, the chemical potential due to the catalytic reaction of the metals are required to evaluate. The platinum and titanium layered electrode on the insulator/semiconductor layered device are fabricated as the test chip. The catalytic reaction of platinum and hydrogen gas were successfully visualized.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We have developed a prototype system of a laser THz emission microscope (LTEM) for inspecting electrical failures in large-scale integrated circuits (LSIs). LTEM measures the THz emission images on LSIs by exciting the p-n junctions with ultra fast laser pulses. Without any electrical probing, we successfully identified interconnection built-in defects such as open or short circuit in C7552 ISCAS'85 benchmark circuits using the prototype system.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Terahertz chemical microscope was carried out to visualize the distribution of the redox reaction of the immobilized-enzyme. The urease films with the spot size of 3, 2, 1 mm in diameter were immobilized on a SiO2/Si/Sapphire layered device and the laser-excited terahertz radiation from the Si layer was observed, which can be related to the chemical potential shift of the SiO2 surface. The redox reaction between the urease and the urea solution was successfully measured.

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Language：English   Publishing type：Research paper (international conference proceedings)  

Terahertz (THz) time-domain spectroscopy (TDS) is attracting more and more attention recently, and has been growing up as a powerful technique for measuring the material parameters. Indium phosphide (InP), which with short carrier average collision time, high band-gap energy and low effective mass, is growing up as one of the best photoconductive materials for emitting and detecting THz waves. An n-type InP of 0.35 Ω•cm was measured with normal transmitted TDS system for the temperature and frequency ranges of 4.2-300 K and 0.2-4 THz, respectively. THz beam was placed in a closed box purged with dry nitrogen gas, and the sample was mounted in a Microstat He cryostat made by Oxford Instruments. Temperature dependence of THz properties for InP as a function of frequency was characterized. © 2009 SPIE.

DOI： 10.1117/12.821683

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Terahertz time-domain spectroscopy (TDS) is advantageous to analyze the absorption spectrum of water since the terahertz electromagnetic waves are absorbed very well. The measurement confirms the absorption coefficient of the pure water increases with the frequency from 0.04 THz to 0.8 THz. Moreover, the absorption increases with an increment of the concentration of Na+ ion in the pure water especially in the low frequency region. © 2009 IEEE.

DOI： 10.1109/ICIMW.2009.5325641

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have explored the low energy charge dynamics of charge-ordered Nd0.5Ca0.5MnO3 and Ba-doped Nd0.5Ca0.48Ba0.02MnO3 manganite thin films by recording the complex optical spectra using terahertz time-domain spectroscopy. The extracted frequency- and temperature-dependent optical conductivity (sigma) and dielectric constant (epsilon(1)) show distinct features at Neel&apos;s ordering and the charge-ordering temperatures (T-N and T-CO). We found that the Ba-doping induced cation disorder effects on sigma and epsilon(1) are more pronounced above T-CO, but these effects are subtle in a large temperature region below T-CO. Both epsilon(1) and sigma increase as a function of temperature and form a scaling relationship epsilon(1)proportional to sigma.

DOI： 10.1063/1.3041638

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Carrier capturing in GaInP/Er,O-codoped GaAs (GaAs:Er,O)/GaInP laser diodes (LDs) was investigated by means of the threshold current density (J(th)) of laser emission. The Er-doped LDs showed laser emission of the GaAs band edge at room temperature, and its J(th) increases with the Er flow rate during the growth. In the dependence of J(th) on reciprocal cavity length, the effective gain factor of the LDs decreased by the codoping of Er and O. These results reveal that ultrafast capturing of injected carriers at an Er-related trap level is accomplished even in the stimulated emission region of GaAs. The fast capturing process allowed us to observe both the spontaneous intra-4f-shell emission due to Er(3+) ions at 1.54 mu m and the stimulated emission at the GaAs band edge (similar to 840 nm) in the Er-doped LDs.

DOI： 10.1063/1.3046784

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have studied the origin of soft-mode hardening in SrTiO(3) thin films using broadband terahertz time-domain spectroscopy. We measured the dielectric dispersions in the terahertz region of as-deposited, O(2) annealed, and high-temperature annealed SrTiO(3) thin films deposited on MgO and La(0.3)Sr(0.7)Al(0.65)Ta(0.35)O(3) substrates. The results show that the ferroelectric soft mode softens dramatically by the high-temperature annealing. We also measure x-ray diffractions and atomic force microscope images and conclude that the hardening of the ferroelectric soft mode in the thin films is determined by the grain size of each crystalline domain which is enlarged by the high-temperature annealing due to remelting. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2991442

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Language：Japanese   Publishing type：Research paper (other academic)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have proposed and demonstrated a novel technique for the noncontact inspection of electrical failures in semiconductor devices using a laser terahertz emission microscope. It was found that the waveforms of the terahertz pulses, emitted by exciting p-n junctions in semiconductor circuits with focused ultrafast laser pulses, depend on the interconnection structures of the circuits. We successfully distinguished damaged silicon metal-oxide-semiconductor field effect transistor circuits with disconnected wires from normal ones by comparing the images of terahertz emission amplitudes between a normal chip and a defective one. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2965810

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Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OPTICAL SOC AMER  

A new type of laser-terahertz emission system for noncontact investigations of chemical solutions has been developed. The system monitors terahertz emission from a sensing plate, which consists of silicon oxide and silicon thin film layers on a sapphire substrate. Sensing of chemical solutions with pH values between 1.68 and 10.01 was demonstrated. The amplitude of the terahertz emission from the sensing plate increased with increasing pH value. This change in the amplitude was caused by a change in the depletion layers of the silicon thin film when protons were adsorbed on the surface of the sensing plate. This study demonstrates that full noncontact monitoring of chemical solutions is possible using the laser-terahertz emission system. (c) 2008 Optical Society of America.

DOI： 10.1364/AO.47.003324

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We have measured the complex dielectric constants of SrTiO3 thin films deposited on MgO substrate, by using the broadband terahertz time-domain spectroscopy. The dielectric dispersion of SrTiO3 thin films with thickness of 1046, 460 and 55 nm has been observed in the frequency range from 0.1 to 8THz. The dispersion mainly consists of the TOI ferroelectric soft mode with a slight absorption of the TO2 phonon mode. From the analysis of the obtained dispersion, we found that the soft mode frequency hardens as the thickness of the film becomes thinner, and is extremely large compared with the bulk crystals. The damping of the soft mode is also larger than that of bulk SrTiO3, which suggests the extrinsic nature of the broadening of the soft mode dispersion. In the thinnest film of 55 nm, even the shape of the dielectric dispersion changes, which may be related to integrated defects near the interface. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jlumin.2007.12.026

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

A recently developed laser terahertz (THz) emission microscope (LTEM) is a new type of inspection tool for semiconductor integrated circuits by 2-D mapping of THz emission excited by femtosecond (fs) laser pulse. For high-resolution imaging, we demonstrate the combination of a hemispherical solid immersion lens (SIL) and an LTEM with transmission-type detection mode. Unlike reflective LTEM geometries, in which both the probing fs laser pulses and the generated THz emissions utilize the same optical components, the transmission system enables flexibility in the construction of both the fs laser and the THz detection optics: The optics on the incident side of the sample are used only to manipulate the fs pulses, while those on the transmission side are meant only for manipulating the THz emission. We could improve the spatial resolution by less than 1.5 mu m by combining the objective lens with the hemispherical SIL with a refractive index it = 1.98 at 780 nm.

DOI： 10.1109/JSTQE.2007.913425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The spin injection and transport properties of low-molecular-weight organic semiconductors such as pentacene and bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT) were investigated utilizing planar-type spin-valve devices with half-metallic La(0.67)Sr(0.33)MnO(3) electrodes. The devices showed clear spin-valve characteristics with a magnetoresistance (MR) ratio of up to 29% at 5 K. The MR ratio was found to depend on the gap spacing of the electrodes, the applied bias voltage, temperature, and the crystallinity of the films. It was also affected by gas adsorption onto the films, indicating that the spins were scattered by carriers and/or radical ions in the films generated through charge transfer from gas molecules. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2905288

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have developed a laser terahertz emission microscope utilizing excitation laser pulses at 1.06 μm wavelength for the inspection and localization of electrical failures in large-scale integrated circuits with multilayered interconnection structures. The system enables to measure terahertz emission images from the backside of a large-scale integrated circuits chip with a multilayered interconnection structure that prevents the observation from the front side. By comparing the terahertz emission images, we successfully distinguish a normal circuit from damaged ones with different positions of the interconnection defects without any electrical probing. © 2009 American Institute of Physics.

DOI： 10.1063/1.3133346

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Ultrafast carrier dynamics and terahertz radiation from Er,O-codoped GaAs (GaAs:Er,O) have been investigated by pump and probe reflectance and time-domain terahertz spectroscopy. In pump and probe reflectance measurements, GaAs:Er,O showed faster relaxation time (0.37-0.56 ps) of photoexcited carriers than undoped GaAs. In terahertz spectroscopy, the radiated terahertz amplitude decreased and the decay time of transient photocurrent became long with increasing Er concentration. The Er concentration dependence was understood by additional electron scattering due to the Er doping. The fast relaxation time and the terahertz radiation properties suggest new applications of GaAs:Er,O for the terahertz frequency region. (C) 2008 American Institute of Physics.

DOI： 10.1063/1.2901025

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Publishing type：Research paper (international conference proceedings)  

Energy-transfer processes in Er,O-codoped GaAs (GaAs : Er,O) have been investigated by means of a pump and probe reflection technique. Time-resolved reflectivity exhibited a characteristic dip; a negative signal due to bandgap renormalization in less than 1 ps and then a gradual recovery. In the recovery process, there were two components, fast and slow. The fast recovery time (several ps) was inversely proportional to Er concentration. The analysis based on a rate equation indicated that it is due to the capture of electrons by charged traps. The slow recovery (30-60 ps) was well coincident with the time (54 ps) predicted theoretically in the framework of a multiphonon-assisted model. Optical excitation cross section of Er ions in GaAs:Er,O has also been studied in various samples and shown to depend strongly on Er concentration. © 2008 WILEY VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/pssa.200776702

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

Energy-transfer processes in Er,O-codoped GaAs (GaAs: Er,O) have been investigated by means of a pump and probe reflection technique. Time-resolved reflectivity exhibited a characteristic dip; a negative signal due to bandgap re-normalization in less than 1 ps and then a gradual recovery. In the recovery process, there were two components, fast and slow. The fast recovery time (several ps) was inversely proportional to Er concentration. The analysis based on a rate equation indicated that it is due to the capture of electrons by charged traps. The slow recovery (30-60 ps) was well coincident with the time (54 ps) predicted theoretically in the framework of a multiphonon-assisted model. Optical excitation cross section of Er ions in GaAs:Er,O has also been studied in various samples and shown to depend strongly on Er concentration. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssa.200776702

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