First-principles calculations for high-harmonic generation in two-dimensional materials: from single atomic layer to thin films*

We present first-principles studies on high-harmonic generation (HHG) in thin materials using the time-dependent density functional theory (TDDFT). HHG in reflected and transmitted waves from a thin film exposed to intense light pulses is modulated from the bulk optical response by light propagation effect and surface effect. We have developed a real-time simulation method combining the time-dependent Kohn-Sham and the Maxwell equations for describing the microscopic dynamics of electrons and electromagnetic field in a thin film. This method can take into account both the propagation and surface effects. Using our method, we investigate the dependencies of HHG on the light intensity, polarization, and film thickness for semiconductor films. We discuss the mechanism of HHG signal growth in reflected and transmitted waves.