X-ray absorption fine-structure (XAFS) spectroscopy is a well-established technique capable of extracting information about a material’s electronic and lattice structure with atomic resolution. While the near-edge region (XANES) of a XAFS spectrum provides information about the electronic configuration, structural information is extracted from the extended XAFS (EXAFS) spectrum, consisting of several hundreds of eV above the absorption edge. With the advent of high harmonic sources, reaching photon energies in soft x-ray (SXR) region, it now becomes possible to connect the spectroscopic capabilities of XAFS to the unprecedented attosecond temporal resolution of a high harmonic source allowing the observation of electronic and lattice dynamics in real time [1,2].
Layered materials, such the transition-metal dichalcogenide TiS2 or graphite, are an emerging class of materials with attractive structural and electronic properties as they can be thinned to a single atomic layer with electron mobilities resembling that of a metal, semiconductor, or semi-metal.
In this work, we utilized broadband water-window-covering attosecond SXR pulses (300 as, ranging from 200
- 550 eV) capable of accessing orbital-specific K- and L-edges of such layered materials to perform transient XAFS
with attosecond time resolution [3,4].
 Teichmann, S. et al, "0.5-keV soft x-ray attosecond continua", Nat. Commun. 7, 11493 (2016).
 Cousin S. et al, "Attosecond streaking in the water window: a new regime of attosecond pulse characterization", Phys. Rev. X, 7, 041030 (2017).
 Buades, B. et al., “Dispersive soft x-ray absorption fine-structure spectroscopy in graphite with an attosecond pulse”, Optica 5 (5), 502 (2018).
 Buades, B. et al., “Attosecond-resolved petahertz carrier motion in semi-metallic TiS2”, arXiv: 1808.06493 (2018).
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.
Ab-initio density functional theory (DFT) has been successful for calculations of ground state properties of various materials. Time-dependent density functional theory (TDDFT) is an extension of the DFT and can describe electron dynamics in molecules, nano-structures, and solids induced by optical electric fields. We have been developing a computational method to describe electron dynamics in a crystalline solid under an irradiation of an ultrashort laser pulse, solving the time-dependent Kohn-Sham equation in real time. The method can be used for an ab-initio description of light-matter interactions. We further couple the electron dynamics calculation with the macroscopic Maxwell equations in a multiscale implementation. It can describe laser pulse propagation in dielectrics and, in particular,the energy transfer from the laser pulse to electrons in dielectrics without any empirical parameters. We apply the method to analyze recent experiments utilizing attosecond spectroscopy methods. We show a few examples. One is for the ultrafast changes of dielectric properties of diamond during the irradiation of an intense few-cycle laser pulse. We mimic the pump-probe measurement employing the multiscale Maxwell + TDDFT simulation. We clarified that the dynamical Franz-Keldysh effect is responsible for the mechanism. The other is to identify the onset of the energy transfer from the laser pulse to SiO_2 when we increase the intensity of the laser pulse. We are currently extending the analysis to obtain a clear and intuitive understanding for the initial stage of laser damage processes.
Laser-induced damage of SiO2 (α-quartz) is investigated by first-principles calculations. The calculations are based on a coupled theoretical framework of the time-dependent density functional theory and Maxwell equation to describe strongly-nonlinear laser-solid interactions. We simulate irradiation of the bulk SiO2 with femtosecond laser pulses and compute energy deposition from the laser pulse to electrons as a function of the distance from the surface. We further analyze profiles of laser-induced craters, comparing the transferred energy with the cohesive energy of SiO2. The theoretical crater profile well reproduces the experimental features for a relatively weak laser pulse. In contrast, the theoretical result fails to reproduce the measured profiles for a strong laser pulse. This fact indicates a significance of the subsequent atomic motions that take place after the energy transfer ends for the formation of the crater under the strong laser irradiation.
We develop a computational approach for interaction between strong laser pulse and dielectrics based on time-dependent density functional theory (TDDFT). In this approach, a key ingredient is a solver to simulate electron dynamics in a unit cell of solids under a time-varying electric field that is a time-dependent extension of the static band calculation. This calculation can be regarded as a constitutive relation, providing macroscopic electric current for a given electric field applied to the medium. Combining the solver with Maxwell equations for electromagnetic fields of the laser pulse, we describe propagation of laser pulses in dielectrics without any empirical parameters. An important output from the coupled Maxwell+TDDFT simulation is the energy transfer from the laser pulse to electrons in the medium. We have found an abrupt increase of the energy transfer at certain laser intensity close to damage threshold. We also estimate damage threshold by comparing the transferred energy with melting and cohesive energies. It shows reasonable agreement with measurements.