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Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
Physical Review B, 93(4), p.045124_1 - 045124_9, 2016/01
Times Cited Count：49 Percentile：86.81(Materials Science, Multidisciplinary)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.
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：84.72(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*; Shinohara, Yasushi*; Otobe, Tomohito; Yabana, Kazuhiro*
Physical Review B, 90(17), p.174303_1 - 174303_8, 2014/11
Times Cited Count：40 Percentile：81.69(Materials Science, Multidisciplinary)We calculate the dielectric response of excited crystalline silicon in electron thermal equilibrium by adiabatic time-dependent density functional theory (TDDFT) to model the response to irradiation by high-intensity laser pulses. We find that the extracted effective mass are in the range of 0.22-0.36 and lifetimes are in the range of 1-14 fs depending on the temperature.
-quartz irradiated with an ultrashort intense laser pulseLee, K.-M.*; Kim, C. M.*; Sato, Shunsuke*; Otobe, Tomohito; Shinohara, Yasushi*; Yabana, Kazuhiro; Jeong, T. M.*
Journal of Applied Physics, 115(5), p.053519_1 - 053519_8, 2014/02
Times Cited Count：27 Percentile：72.54(Physics, Applied)A computational method based on a first-principles multiscale simulation has been used for calculating the optical response and the ablation threshold of an optical material irradiated with an ultrashort intense laser pulse. The method was applied to investigate the changes in the optical reflectance of quartz bulk, half-wavelength thin-film and quarter-wavelength thin-film and to estimate their ablation thresholds. Despite the adiabatic local density approximation used in calculating the exchange-correlation potential, the reflectance and the ablation threshold obtained from our method agree well with the previous theoretical and experimental results. The method can be applied to estimate the ablation thresholds for optical materials in general.
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：89.86(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：67.67(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.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
no journal, ,
We calculate the dynamical Franz-Keldysh effect employing the time-dependent density-functional theory by solving time-dependent Kohn-Sham equation with real-time method for the first time. Our results reproduce previous theoretical and experimental result qualitatively. We also calculate the time evolution of the dielectric function in the intense mid-infrared laser. The modulation of the dielectric function is defined by the temporal intensity of the laser field and the adiabatic perspective is valid for intense laser field.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
no journal, ,
We calculate the dynamical Franz-Keldysh effect employing the time-dependent density-functional theory by solving time-dependent Kohn-Sham equation with real-time method for the first time. Our results reproduce previous theoretical and experimental result qualitatively. We also calculate the time evolution of the dielectric function in the intense mid-infrared laser. The modulation of the dielectric function is defined by the temporal intensity of the laser field and the adiabatic perspective is valid for intense laser field.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
no journal, ,
We calculate the modulation of the optical properties of a diamond under an intense static fields and intense mid-infrared (MIR) light fields employing the time-dependent density functional theory (TDDFT). We solve the time-dependent Khon-Sham equation with real-time and -space method under time-dependent vector fields. Our results well reproduce the exponentially tail of absorption below the band gap and the Franz-Keldysh oscillation above the band gap.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
no journal, ,
We calculate the modulation of the optical properties of a diamond under the intense static fields and intense mid-infrared (MIR) light fields employing the time-dependent density functional theory (TDDFT). We solve the time-dependent Khon-Sham equation with real-time and -space method under time-dependent vector fields. Our results reproduce the exponentially tail of absorption below the band gap and the Franz-Keldysh oscillation above the band gap. We found that the absorption below the gap oscillates with the electric field and the adiabatic approximation is valid for higher laser intensity.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
no journal, ,
The electronic states in dielectrics are modulated by an intense THz light. One of the most illuminative phenomena is the dynamical Franz-Keldysh effect (DFKE). We calculate the DFKE with first peinciple method, time-dependent density functional theory. Our result shows that the change of the dielectric function has the peak before the peak of the light field.
Otobe, Tomohito; Shinohara, Yasushi*; Sato, Shunsuke*; Yabana, Kazuhiro*
no journal, ,
We present the first-principle real-time simulation for the modulation of the dielectric functionunder an intense mid-infrared coherent light, dynamical Franz-Keldysh effect (DFKE), of the diamond. Our result shows that the change of the dielectric function is large at the minimum of the pump light field at low intensity region, and the phase shift of the DFKE signals occurs as the pump light field increases. We ascribe this phase shift to the competitive effect of multiphoton processes.
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.
