Nowadays ab-initio calculations are recognized as an essential and indispensable tool in materials science. Although density functional theory has been widely used, it is a theory for electronic ground states. To describe electronic excitations and dynamics, time-dependent density functional theory (TDDFT) has been developed. Solving the time-dependent Kohn-Sham equation, the basic equation of the TDDFT, in real time, it has been possible to explore ultrafast electron dynamics induced by ultrashort laser pulses with typical resolutions of 0.02 nm in space and 1 as in time.
We are developing a novel ab-initio simulation method to describe a propagation of ultrashort laser pulses in a bulk medium based on the TDDFT. A key innovation in our simulation method is the multiscale combination of simulations in two different scales, electromagnetic field analysis for the propagation of pulsed light and the TDDFT calculation for the electron dynamics in atomic scale induced by the pulsed light. Our method allows us to describe interactions between an ultrashort laser pulse and bulk materials without any empirical parameters, in particular the energy transfer from the pulsed light to electrons in the medium. The energy transfer is significant in practical usages of the pulsed light, for example, to understand the initial stage of non-thermal laser processing. Our method provides a useful platform of numerical experiments that faithfully describe optical experiments such as pump-probe measurements. We believe that the simulation method will contribute much to progresses in wide fields of optical sciences.
We apply the method to interactions between an intense and ultrashort pulsed light and nanoscale semiconducting materials: silicon nanofilms and silicon 3D nanostructures. Under the irradiation of the intense pulsed light, our calculations indicate that the optical properties of the silicon changes from insulator to metal, owing to the multi-photon carrier excitations. For a propagation of a pulsed light in silicon nanofilms, we solve a coupled problem of 1D-Maxwell equations for the electromagnetic fields of the pulsed light and 3D electron dynamics described by the time-dependent Kohn-Sham equation. Penetrating the silicon nanofilms, the waveform of the pulsed light is found to be modulated during the propagation in the film: suppression in the high intensity amplitude, distortion in the tail part, and so on. A collaboration with an experimental research group is ongoing on this subject.
As 3D silicon nanostructures, we consider two cases: a nanospheres of about 500 nm diameter in which a focusing of pulsed light takes place, and a bowtie-shaped nanogap composed of square nanoblocks of about 400 nm side in which a near field enhancement is expected. For the strong intensity beam, the spatial distribution of the energy transfer is modulated by the carrier excitation induced by the focused light, and it decreases the lifetime of the light confinement.
Mitsuharu Uemoto and Kazuhiro Yabana, "Ab-initio simulation for propagation of ultrashort laser pulse in solids (Conference Presentation)," Proc. SPIE 10673, Advances in Ultrafast Condensed Phase Physics, 106730S (Presented at SPIE Photonics Europe: April 24, 2018; Published: 29 May 2018); https://doi.org/10.1117/12.2306727.5788736091001.
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