Tomohito Otobe

Time-dependent density functional theory for strong electromagnetic fields in crystalline solids

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 ~ 10^13 W/cm^2, 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 3 \times 1012 W/cm2, there is a small decrease. At higher intensities, above 2 \times 10^13 W/cm^2, the reflectivity increases strongly. This behavior can be understood qualitatively in a model treating the excited electron-hole pairs as a plasma.

First-principles calculation of the electron dynamics in crystalline SiO2

We present a first-principles description for electron dynamics in crystalline SiO(2) induced by an optical field in both weak and intense regimes. We rely upon the time-dependent density-functional theory with the adiabatic local-density approximation, and a real-space and real-time method is employed to solve the time-dependent Kohn-Sham equation. The response calculation to a weak field provides us with information on the dielectric function, while the response to an intense field shows the optical dielectric breakdown. We discuss the critical threshold for the dielectric breakdown of crystalline SiO(2), in comparison with the results for diamond.

Numerical pump-probe experiments of laser-excited silicon in nonequilibrium phase

S.A. Sato, K. Yabana, 1 Y. Shinohara, T. Otobe, and G.F. Bertsch Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8577, Japan Advanced Photon Research Center, JAEA, Kizugawa, Kyoto 619-0615, Japan Department of Physics and Institute for Nuclear Theory, University of Washington, Seattle 98195, U.S.A.

Femtosecond time-resolved dynamical Franz-Keldysh effect

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.

Time-dependent density functional theory of high-intensity short-pulse laser irradiation on insulators

We calculate the energy deposition by very short laser pulses in SiO_2 (alpha-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. For the most realistic simulations we employ a meta-GGA Kohn-Sham potential similar to that of Becke and Johnson. 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 for a range of intensities from below the melting threshold to well beyond the ablation threshold. 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.

Femtosecond laser ablation dynamics of fused silica extracted from oscillation of time-resolved reflectivity

Femtosecond laser ablation dynamics of fused silica is examined via time-resolved reflectivity measurements. After optical breakdown was caused by irradiation of a pump pulse with fluence F{sub pump} = 3.3–14.9 J/cm{sup 2}, the reflectivity oscillated with a period of 63 ± 2 ps for a wavelength λ = 795 nm. The period was reduced by half for λ = 398 nm. We ascribe the oscillation to the interference between the probe pulses reflected from the front and rear surfaces of the photo-excited molten fused silica layer. The time-resolved reflectivity agrees closely with a model comprising a photo-excited layer which expands due to the formation of voids, and then separates into two parts, one of which is left on the sample surface and the other separated as a molten thin layer from the surface by the spallation mechanism. Such oscillations were not observed in the reflectivity of soda-lime glass. Whether the reflectivity oscillates or not probably depends on the layer viscosity while in a molten state. Since viscosity of the molten fused silica is several orders of magnitude higher than that of the soda-lime glass at the same temperature, fused silica forms a molten thin layer that reflects the probe pulse, whereas the soda-lime glass is fragmented into clusters.

Nonadiabatic generation of coherent phonons.

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 produced by intense laser pulses. We examine the process in the crystalline semimetal antimony (Sb), where nonadiabatic coupling is very important. This material is of particular interest because it exhibits strong phonon coupling and optical phonons of different symmetries can be observed. The TDDFT is able to account for a number of qualitative features of the observed coherent phonons, despite its unsatisfactory performance on reproducing the observed dielectric functions of Sb. A simple dielectric model for nonadiabatic coherent phonon generation is also examined and compared with the TDDFT calculations.

First-principles calculations for multiphoton absorption in α-quartz under intense short laser irradiation.

We present a first-principles description for the electron dynamics and multiphoton absorption in crystalline SiO(2) induced by an optical field. We rely upon time-dependent density-functional theory with adiabatic local-density approximation, and a real-space and real-time method is employed to solve the time-dependent Kohn-Sham equation. Computational results show the absorption of photons to be in excess of the minimum required for electron excitation from the valence band to the conduction band. This is similar to the above threshold ionization in atoms and molecules.

Dielectric response of laser-excited silicon at finite electron temperature

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. The real part of the dielectric function is characterized by the strong negative behavior at low frequencies due to excited electron-hole pairs. The response agrees rather well with the numerical pump-probe calculations which simulate electronic excitations in nonequilibrium phase immediately after the laser pulse irradiation. The thermal response is also compared with the Drude model which includes electron effective mass and collision time as fitting parameters. We find that the extracted effective masses are in the range of 0.22-0.36 and lifetimes are in the range of 1-14 fs depending on the temperature. The short Drude lifetimes show that strong damping is possible in the adiabatic TDDFT, despite the absence of explicit electron-electron collisions.

First-principles simulation of the optical response of bulk and thin-film α-quartz irradiated with an ultrashort intense laser pulse

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 employs Maxwells equations to describe laser pulse propagation and time-dependent density functional theory to describe the generation of conduction band electrons in an optical medium. Optical properties, such as reflectance and absorption, were investigated for laser intensities in the range 1010 W/cm2 to 2 × 1015 W/cm2 based on the theory of generation and spatial distribution of the conduction band electrons. The method was applied to investigate the changes in the optical reflectance of α-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 m...