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Sato, Shunsuke*; Yabana, Kazuhiro*; Shinohara, Yasushi*; Otobe, Tomohito; Lee, K.-M.*; Bertsch, G. F.*
Physical Review B, 92(20), p.205413_1 - 205413_6, 2015/11
Times Cited Count：46 Percentile：84.85(Materials Science, Multidisciplinary)We calculate the energy deposition by very short laser pulses in SiO
(
-quartz) with a view to establishing systematics for predicting damage and nanoparticle production. The theoretical framework is time-dependent density functional theory, implemented by the real-time method in a multiscale representation. We find that the deposited energy in the medium can be accurately modeled as a function of the local electromagnetic pulse fluence. The energy deposition function can in turn be quite well fitted to the strong-field Keldysh formula. We find reasonable agreement between the damage threshold and the energy required to melt the substrate. The ablation threshold estimated by the energy to convert the substrate to an atomic fluid is higher than the measurement, indicating significance of nonthermal nature of the process. A fair agreement is found for the depth of the ablation.
Sato, Shunsuke*; Yabana, Kazuhiro; Shinohara, Yasushi*; Otobe, Tomohito; Bertsch, G. F.*
Physical Review B, 89(6), p.064304_1 - 064304_8, 2014/02
Times Cited Count：62 Percentile：89.98(Materials Science, Multidisciplinary)We calculate the dielectric response of crystalline silicon following irradiation by a high-intensity laser pulse, modeling the dynamics by the time-dependent Kohn-Sham equations in the presence of the laser field. As expected, the excited silicon shows features of an electron-hole plasma of nonequilibrium phase in its response, characterized by a negative divergence in the real part of the dielectric function at small frequencies. We also find that the imaginary part of the dielectric function can be negative, particularly for the parallel polarization of pump and probe fields.
Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*; Iwata, Junichi*; Otobe, Tomohito; Bertsch, G. F.*
Journal of Chemical Physics, 137(22), p.22A527_1 - 22A527_8, 2012/08
Times Cited Count：25 Percentile：67.85(Chemistry, Physical)The time-dependent density functional theory (TDDFT) is the leading computationally feasible theory to treat excitations by strong electromagnetic fields. Here the theory is applied to coherent optical phonon generation. We examine the process in the crystalline semimetal antimony (Sb), where nonadiabatic coupling and optical phonon of different symmetries can be observed. The TDDFT is able to account for a number of qualitative features of the observed coherent phonon.
Yabana, Kazuhiro*; Sugiyama, Takeshi*; Shinohara, Yasushi*; Otobe, Tomohito; Bertsch, G. F.*
Physical Review B, 85(4), p.045134_1 - 045134_11, 2012/01
Times Cited Count：187 Percentile：97.94(Materials Science, Multidisciplinary)We apply the coupled dynamics of time-dependent density functional theory and Maxwell equations to the interaction of intense laser pulses with crystalline silicon. As a function of electromagnetic field intensity, we see several regions in the response. At the lowest intensities, the pulse is reflected and transmitted in accord with the dielectric response, and the characteristics of the energy deposition is consistent with two-photon absorption. The absorption process begins to deviate from that at laser intensities 
10
W/cm
, where the energy deposited is of the order of 1 eV per atom. Changes in the reflectivity are seen as a function of intensity. When it passes a threshold of about 
W/cm, there is a small decrease. At higher intensities, above 2
10 W/cm, the reflectivity increases strongly. This behavior can be understood qualitatively in a model treating the excited electron-hole pairs as a plasma.
Yabana, Kazuhiro*; Shinohara, Yasushi*; Otobe, Tomohito; Iwata, Junichi*; Bertsch, G. F.*
Procedia Computer Science, 4, p.852 - 859, 2011/00
Times Cited Count：2 Percentile：60.86(Computer Science, Theory & Methods)We report a first-principle computational method to describe many-electron dynamics in crystalline solid. The method is based on the time-dependent density functional theory, solving time-dependent Kohn-Sham equation in real-time and real-space. The calculation is efficiently parallelized by distributing computations of different 
-point among processors. To illustrate usefulness of the method and efficiency of the parallel computation, we show calculations of electron dynamics in bulk Si induced by intense and ultrafast laser pulse.
Shinohara, Yasushi*; Yabana, Kazuhiro*; Kawashita, Yosuke*; Iwata, Junichi*; Otobe, Tomohito; Bertsch, G. F.*
Physical Review B, 82(15), p.155110_1 - 155110_10, 2010/10
Times Cited Count：63 Percentile：88.63(Materials Science, Multidisciplinary)We apply the adiabatic time-dependent density functional theory (TDDFT) to the generation of coherent optical phonons in Si crystals by intense laser pulses. The theory reproduces the main phenomena observed experimentally: dependence on polarization, strong growth at the direct band gap, and the change in phase from below to above the band gap. Both show that two mechanisms invoked in phenomenological theory, namely, impulsively stimulated Raman scattering and displacive excitation, are present in the TDDFT. The calculated phase of the coherent phonon is in qualitative agreement with experiment and with phenomenological modeling in the vicinity of the direct band gap. At higher laser frequencies, the TDDFT predicts additional structure not present in the modeling.
Shinohara, Yasushi*; Kawashita, Yosuke*; Iwata, Junichi*; Yabana, Kazuhiro*; Otobe, Tomohito; Bertsch, G. F.*
Journal of Physics; Condensed Matter, 22(38), p.384212_1 - 384212_4, 2010/09
Times Cited Count：13 Percentile：50.83(Physics, Condensed Matter)We report a first-principle description for coherent phonon generation in diamond based on the time-dependent density functional theory. The time-dependent Kohn-Sham equation is solved in real-time to calculate the electron dynamics in periodic solid under an ultrashort laser pulse. We find the calculated forces acting on ions are consistent with measurements in selection rule and in dependence on laser intensity.
Otobe, Tomohito; Yamagiwa, Mitsuru; Iwata, Junichi*; Yabana, Kazuhiro*; Nakatsukasa, Takashi*; Bertsch, G. F.*
Physical Review B, 77(16), p.165104_1 - 165104_5, 2008/04
Times Cited Count：154 Percentile：96.78(Materials Science, Multidisciplinary)We present a first-principle calculation for optical dielectric breakdown induced by an intense laser field. We employ the time-dependent density-functional theory, solving the time-dependent Kohn-Sham equation in real time and real space. The calculation shows a qualitative change of electron dynamics as the laser intensity increase, above 710
W/cm. Following the pulse, the excited electron exhibit a coherent plasma oscillation.
Otobe, Tomohito; Yamagiwa, Mitsuru; Iwata, Junichi*; Yabana, Kazuhiro*; Nakatsukasa, Takashi*; Bertsch, G. F.*
RIKEN Accelerator Progress Report, Vol.41, P. 185, 2008/00
We simulate the optical breakdown process of dielectrics under the intense laser field employing the first-principle method. Our result for the diamond, which is typical dielectrics, shows the saturation of the energy absorption and electron excitation. We found also the induced field by the surface charge becomes out of phase to the applied field.
Otobe, Tomohito; Yamagiwa, Mitsuru; Yabana, Kazuhiro*; Iwata, Junichi*; Nakatsukasa, Takashi*; Bertsch, G. F.*
no journal, ,
no abstracts in English
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro; Bertsch, G. F.*
no journal, ,
We calculate the dynamical Franz-Keldysh effect (DFKE), the ultrafast change of the dielectric function by an intense laser field, employing the time-dependent density functional theory. Our result show that the time for the peak of the change of the dielectric function dose not coincide with the peak of the electric field. This difference decreases as the laser intensity increases. To explore this effect analytically, we construct the new time-resolved formula for DFKE. This formula reproduces our numerical results qualitatively.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro; Bertsch, G. F.*
no journal, ,
no abstracts in English
Otobe, Tomohito; Yabana, Kazuhiro*; Shinohara, Yasushi*; Sato, Shunsuke*; Bertsch, G. F.*
no journal, ,
We revise the familiar Keldysh formula for the solid states in a simpler formulation. Our formula unable us to analyze the 
-th photon process and the distribution of the electron-hole pair in Bloch phase space.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*; Bertsch, G. F.*
no journal, ,
We theoretically investigate the dynamical Franz-Keldysh effect in femtosecond time resolution, that is, the time-dependent modulation of a dielectric function at around the band gap under an irradiation of an intense laser field. We develop a pump-probe formalism in two distinct approaches: first-principles simulation based on real-time time-dependent density functional theory and analytic consideration of a simple two-band model. We find that, while time-average modulation may be reasonably described by the static Franz-Keldysh theory, a remarkable phase shift is found to appear between the dielectric response and the applied electric field.
