Faculty Profiles - JESCHKE HARALD OLAF

Laser ablation thresholds of silicon for different pulse durations: theory and experiment Reviewed

HO Jeschke, ME Garcia, M Lenzner, J Bonse, J Kruger, W Kautek

APPLIED SURFACE SCIENCE 197 839 - 844 2002.9

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

The ultrafast laser ablation of silicon has been investigated experimentally and theoretically. The theoretical description is based on molecular dynamics (MD) simulations combined with a microscopic electronic model. We determine the thresholds of melting and ablation for two different pulse durations tau = 20 and 500 fs. Experiments have been performed using 100 Ti:Sapphire laser pulses per spot in air environment. The ablation thresholds were determined for pulses with a duration of 25 and 400 fs, respectively. Good agreement is obtained between theory and experiment. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0169-4332(02)00458-0

Theoretical description of the ultrafast ablation of diamond and graphite: dependence of thresholds on pulse duration Reviewed

HO Jeschke, ME Garcia

APPLIED SURFACE SCIENCE 197 107 - 113 2002.9

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

A theoretical description of the ultrafast ablation of diamond and graphite is presented. Laser induced lattice deformations and melting are described with the help of molecular dynamics simulations on time dependent potential energy surfaces derived from a microscopic electronic Hamiltonian. Thermalization effects are explicitly taken into account. We calculate the ablation thresholds as a function of the pulse duration for femtosecond pulses. For both materials we obtain smoothly increasing thresholds for increasing duration. The damage and ablation mechanisms are discussed. (C) 2002 Elsevier Science B.V All rights reserved.

DOI： 10.1016/S0169-4332(02)00312-4

Time-dependent energy absorption changes during ultrafast lattice deformation Reviewed

HO Jeschke, ME Garcia, KH Bennemann

JOURNAL OF APPLIED PHYSICS 91 ( 1 ) 18 - 23 2002.1

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

The ultrafast time dependence of the energy absorption of covalent solids upon excitation with femtosecond laser pulses is theoretically analyzed. We use a microscopic theory to describe laser induced structural changes and their influence on the electronic properties. We show that from the time evolution of the energy absorbed by the system important information on the electronic and atomic structure during ultrafast phase transitions can be gained. Our results reflect how structural changes affect the capability of the system to absorb external energy. (C) 2002 American Institute of Physics.

DOI： 10.1063/1.1423397

Nonthermal fragmentation of C-60 Reviewed

HO Jeschke, ME Garcia, JA Alonso

CHEMICAL PHYSICS LETTERS 352 ( 3-4 ) 154 - 162 2002.1

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

A theoretical study of the subpicosecond fragmentation of C-60 clusters in response to ultrafast laser pulses is presented. We simulate the laser excitation and the consequent nonequilibrium relaxation dynamics of the electronic and nuclear degrees of freedom. The first stages of the nonequilibrium dynamics are dominated by a breathing mode followed by the cold ejection of single C atoms, in contrast to the dimer emission which characterizes the thermal relaxation. We also determine the nonequilibrium damage thresholds as a function of the pulse duration. 0 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(01)01453-1

Theory for the ultrafast ablation of graphite films Reviewed

HO Jeschke, ME Garcia, KH Bennemann

PHYSICAL REVIEW LETTERS 87 ( 1 ) 2001.7

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

The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser-induced structural instabilities. For high absorbed energies (>3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold (>2.0 eV/atom) ablation occurs via removal of intact graphite sheets.

DOI： 10.1103/PhysRevLett.87.015003

Femtosecond laser-induced ablation of graphite Reviewed

K Sokolowski-Tinten, S Kudryashov, Temnov, V, J Bialkowski, D von der Linde, A Cavalleri, HO Jeschke, ME Garcia, KH Bennemann

ULTRAFAST PHENOMENA XII 66 425 - 427 2001

Language：English Publishing type：Research paper (international conference proceedings) Publisher：SPRINGER-VERLAG BERLIN

The dynamics of fs-laser ablation of graphite has been investigated experimentally and theoretically. The experimental observation of two different ablation mechanisms is supported by molecular dynamics calculations, which incorporate the changes of the interatomic potentials due to electronic excitation.

DOI： 10.1007/978-3-642-56546-5_124

Theory for the ultrafast dynamics of excited clusters: interplay between elementary excitations and atomic structure Reviewed

ME Garcia, HO Jeschke, Grigorenko, I, KH Bennemann

APPLIED PHYSICS B-LASERS AND OPTICS 71 ( 3 ) 361 - 371 2000.9

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

We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses. We analyze the excitation of different types of clusters (Hg-n, Ag-n, Si-n C-60 and Xe-n) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse.
For low-intensity pulses (I < 10(12) W/cm(2)) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Si-n and C-60 clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy.
As an example for the case of high intensities (I > 10(15) W/cm(2)), the explosion of clusters under the action of very intense ultrashort laser fields is described.

DOI： 10.1007/s003400000356

Theory for laser-induced ultrafast phase transitions in carbon Reviewed

HO Jeschke, ME Garcia, KH Bennemann

APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 69 ( 7 ) S49 - S53 1999.12

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

The response of carbon to femtosecond laser pulses of arbitrary form, different durations, and different intensities is studied theoretically. We perform molecular dynamics simulations based on a microscopic electronic Hamiltonian. We include in our model the theoretical description of the pulse form, the electron thermalization, and diffusion effects explicitly. We apply our method to diamond and C-60 crystals. For the diamond case, we show that a femtosecond laser pulse induces a nonequilibrium transition to graphite, which takes place for a wide range of pulse durations and intensities. This ultrafast collective motion of the atoms occurs within a time scale shorter than 100 fs. The laser-induced melting of a C-60 crystal under pressure is also analyzed. In this case, an ultrafast melting of the system occurs. We discuss the mechanisms underlying these nonequilibrium phase transitions.

DOI： 10.1007/s003390051354

Microscopic analysis of the laser-induced femtosecond graphitization of diamond Reviewed

HO Jeschke, ME Garcia, KH Bennemann

PHYSICAL REVIEW B 60 ( 6 ) R3701 - R3704 1999.8

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

We present a theoretical study of ultrafast phase transitions induced by femtosecond laser pulses of arbitrary form. Molecular-dynamics simulations on time dependent potential-energy surfaces derived from a microscopic Hamiltonian are performed. Applying this method to diamond, we show that a nonequilibrium transition to graphite takes place for a wide range of laser pulse durations and intensities. This ultrafast transition (similar to 100 fs) is driven by the suppression of the diamond minimum in the potential-energy surface of the laser excited system. [S0163-1829(99)50730-3].

DOI： 10.1103/PhysRevB.60.R3701

Theory for the ultrafast structural response of optically excited small clusters: Time dependence of the ionization potential Reviewed

HO Jeschke, ME Garcia, KH Bennemann

PHYSICAL REVIEW A 54 ( 6 ) R4601 - R4604 1996.12

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

Combining an electronic theory with molecular-dynamics simulations we present results for the ultrafast structural changes in small clusters. We determine the time scale for the change from the linear to a triangular structure after the photodetachment process Ag-3(-) --> Ag-3. We show that the time-dependent change of the ionization potential reflects in detail the internal degrees of freedom, in particular coherent and incoherent motion, and that it is sensitive to the initial temperature. We compare with experiment and point out the general significance of our results.

DOI： 10.1103/PhysRevA.54.R4601

Analysis of the ultrafast dynamics of the silver trimer upon photodetachment Reviewed

HO Jeschke, ME Garcia, KH Bennemann

JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS 29 ( 14 ) L545 - L549 1996.7

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

We study the ultrafast dynamics of Ag-3 clusters immediately after the photodetachment of Ag-3(-) by performing molecular dynamics simulations based on a microscopic electronic theory. We determine the time scale for the change from the linear (initial situation) to a triangular structure (potential minimum) of the ground state of Ag-3. Our results are in good agreement with experiment. We show how the time dependence of the internal degrees of freedom is reflected in the 'pump and probe' signal. Furthermore, we discuss the application of our results to magnetic systems and chemical reactions.

DOI： 10.1088/0953-4075/29/14/007