掲載種別：研究論文（学術雑誌）   出版者・発行元：Optica Publishing Group  

To realize practical applications of terahertz (THz) waves, it is crucial to establish high-speed and real-time spectroscopic techniques. Although single-shot spectroscopy based on THz time-domain spectroscopy (THz-TDS) has been extensively studied, reports on fast spectroscopy using wavelength-tunable THz sources remain limited. In this paper, we overview the characteristics of various wavelength-tunable sources and introduce recent advancements in high-speed and single-shot THz spectroscopy based on parametric sources. Three approaches are explored: (1) high-speed wavelength sweeping and switching, (2) simultaneous multi-wavelength generation, and (3) quasi-single-shot spectroscopy using pulse trains. Additionally, we present a real-time sample identification method using machine learning, applicable to all three approaches. This enables real-time spectral identification and visualization of reagent distributions with high dynamic range. Our methodology is expected to extend the applicability of THz spectroscopy to new domains that have been challenging for conventional systems.

DOI： 10.1364/ao.570731

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その他リンク： https://opg.optica.org/viewmedia.cfm?URI=ao-64-30-F32&seq=0

担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tthz.2025.3592354

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

DOI： 10.1038/s41598-025-06264-7

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その他リンク： https://www.nature.com/articles/s41598-025-06264-7

掲載種別：研究論文（学術雑誌）   出版者・発行元：AIP Publishing  

Improving the repetition rate of a pulsed laser is an important issue in measurement and processing, as it leads to increased average power. However, in general, the heat generated by high-power lasers that use Q-switches cannot be ignored as the repetition rate increases, limiting the repetition rate. Therefore, in this study, we propose a new method of constructing a cavity in which an optical amplifier is inserted separately from the laser resonator, for generating a laser pulse train inside that cavity. By injecting a single laser pulse into this cavity, we were able to create a pulse train of up to ∼50 pulses. In addition, we confirmed that using the generated pulse train as the pump beam for a terahertz parametric source is useful to achieve a high average output. By selecting the appropriate amplifier inside the cavity, it is possible to create a pulse train from any wavelength or pulse width of the laser beam. This method makes it easier to achieve high average power from lasers when increasing the repetition rate is difficult and will contribute to the expansion of laser applications.

DOI： 10.1063/5.0261964

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担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optica Publishing Group  

We have conducted spectroscopy of broadband terahertz (THz) pulses produced by femtosecond lasers via THz parametric detection. Conventional techniques for detecting broadband THz-waves have predominantly relied on time-domain waveform sampling through THz time-domain spectroscopy (THz-TDS), where challenges persist in achieving real-time performance and optimal frequency resolution. To address these issues, we have implemented THz parametric detection, wherein THz-waves are converted to near-infrared light, facilitating one-shot spectroscopy of broadband THz-waves. In addition, the frequency resolution of this method has been improved by adopting a non-collinear phase matching condition.

DOI： 10.1364/optica.538030

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その他リンク： https://opg.optica.org/viewmedia.cfm?URI=optica-12-2-239&seq=0

担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optica Publishing Group  

Terahertz (THz) parametric detection is a highly sensitive method that upconverts a THz wave into a near-infrared beam for detection. Lithium niobate has primarily been used as the nonlinear optical crystal in this approach. However, the frequency band with high parametric gain is limited, leading to increasing interest in other nonlinear optical crystals. Here, we demonstrate that lithium tantalate provides high-sensitivity detection at low frequencies, achieving a maximum improvement of 15 dB over the 0.8–1.5 THz band.

DOI： 10.1364/ol.544632

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その他リンク： https://opg.optica.org/viewmedia.cfm?URI=ol-50-2-527&seq=0

担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP Publishing  

Abstract

We demonstrated the bandwidth broadening of terahertz waves detected by heterodyne electro-optical sampling by implementing a ridge waveguide structure in a lithium niobate (LiNbO₃) crystal. Such an approach effectively reduces absorption loss, eases the phase matching condition and enhances the nonlinear interaction length through the optical confinement effect. As a result, we have more than doubled the bandwidth and improved the signal-to-noise ratio compared with an equivalent approach in a bulk LiNbO₃ crystal. Heterodyne electro-optic sampling in a ridged-waveguide structure is only marginally dependent on the probe beam wavelength, suggesting its potential as a versatile method for broadband terahertz detection.

DOI： 10.35848/1882-0786/ad3799

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その他リンク： https://iopscience.iop.org/article/10.35848/1882-0786/ad3799/pdf

担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：AIP Publishing  

In a conventional injection-seeded terahertz (THz) parametric generator (is-TPG), a complicated achromatic optical system controlling the angle of incidence of the seed beam is used to ensure tunability because both the pump and seed beams must satisfy non-collinear phase-matching conditions. In this study, we found that a THz output and tunability similar to those characteristics of a conventional is-TPG can be obtained even when the pump and seed beams are coaxially injected into the LiNbO3 crystal. In this new generation mechanism, a weak THz-wave is generated by the difference frequency of optical mixing between the pump and seed beams at the Cherenkov phase-matching angle and then strongly parametrically amplified. Thus, we describe our device as a collinear is-TPG. Simplicity is improved by eliminating any need for achromatic optics.

DOI： 10.1063/5.0182369

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担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

DOI： 10.1007/s10762-023-00962-x

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その他リンク： https://link.springer.com/article/10.1007/s10762-023-00962-x/fulltext.html

掲載種別：研究論文（学術雑誌）   出版者・発行元：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/jstqe.2023.3296989

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掲載種別：研究論文（学術雑誌）   出版者・発行元：Springer Science and Business Media LLC  

Abstract

In this study, we propose a technique for identifying and imaging reagents through shielding over a wide dynamic range using a real-time terahertz (THz) spectroscopy system with multi-wavelength THz parametric generation/detection and machine learning. To quickly identify reagents through shielding, the spectral information of the “detection Stokes beam” is used for reagent recognition via machine learning. In general THz wave-based reagent identification, continuous spectra are acquired and analyzed quantitatively by post-processing. In actual applications, however, such as testing for illicit drugs in mail, the technology must be able to quickly identify reagents as opposed to quantifying the amount present. In multi-wavelength THz parametric generation/detection, THz spectral information can be measured instantly using a “multi-wavelength detection Stokes beam” and near-infrared (NIR) camera. Moreover, machine learning enables reagent identification in real-time and over a wide dynamic range. Furthermore, by plotting the identification results as pixel values, the spatial distribution of reagents can be imaged at high speed without the need for post-processing.

DOI： 10.1038/s41598-023-40013-y

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その他リンク： https://www.nature.com/articles/s41598-023-40013-y

担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Institute of Electrical and Electronics Engineers (IEEE)  

DOI： 10.1109/tthz.2022.3231965

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：MDPI AG  

In this study, we developed a multi-wavelength terahertz-wave parametric generator that operates with only one injection seeding laser. Tunable lasers used as an injection seeder must be single-frequency oscillators, and conventional multi-wavelength terahertz-wave parametric generator requires basically the same number of lasers as the number of wavelengths. In order to solve this problem, we developed a new external cavity semiconductor laser that incorporates a DMD in its wavelength-selective mechanism. In this process, stable multi-wavelength oscillation from a single laser was made possible by efficiently causing four-wave mixing. This seed laser can be applied to practical real-time terahertz spectroscopy by arbitrarily switching the desired wavelength to be generated and the interval between multiple wavelengths.

DOI： 10.3390/photonics9040258

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担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optica Publishing Group  

We achieved noise-free terahertz (THz)-wave output from an injection-seeded THz-wave parametric generator (is-TPG) employing high-power injection seeding. A conventional is-TPG uses a weak continuous-wave (CW) seed beam. The position in which broadband noise is generated (via spontaneous parametric down-conversion) and the position of the THz signal overlap. Thus, the output features broadband TPG noise, reducing the signal-to-noise ratio. To solve this problem, we shifted the position in which the THz signal is generated to the front of the crystal; we separated the signal from broadband TPG noise using a high-powered, pulsed seed beam that was 10⁷-fold more powerful than the CW seed beam. Thus, we extracted only the THz signal; we achieved a noise-free is-TPG. This system features a signal-to-noise ratio of 95 dB, approximately 40 dB better than the signal-to-noise ratio of the conventional system.

DOI： 10.1364/ol.448636

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担当区分：筆頭著者,　責任著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optica Publishing Group  

In this study, we demonstrate real-time terahertz (THz) spectroscopy using a rapidly wavelength-switchable injection-seeded THz parametric generator. We developed a wavelength-switchable external cavity diode laser using a digital micromirror device as a seed source for the generator. We realized fast acquisition of THz spectra by switching the wavelength of the laser for each pump beam pulse. This system can rapidly switch wavelengths and easily increase the number of measurement wavelengths, and it also has a wide dynamic range, of more than 75 dB, and high stability. Furthermore, by combining this system with THz parametric detection, all wavelengths can be detected in a single frame using a near infrared camera for real-time reagent measurement.

DOI： 10.1364/ol.423985

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担当区分：筆頭著者   掲載種別：研究論文（学術雑誌）   出版者・発行元：Optica Publishing Group  

We developed a high-power amplified spontaneous emission (ASE)-free fast wavelength-switchable external cavity diode laser (ECDL) using a digital micromirror device (DMD) as the wavelength selector. Generally, with a conventional fast wavelength-switchable ECDL with a DMD, the output power is limited by the damage threshold of the DMD. However, with our ECDL, a high-power output was realized by optimizing the beam focus on the DMD. In addition, an ASE-free stable output was realized through the introduction of a ring cavity. As a result, we successfully developed a fast wavelength-switchable ECDL realizing a high-power ASE-free output of over 300 mW.

DOI： 10.1364/ao.416033

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