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First-principles calculations for determining the thickness to maximize HHG efficiency of laser-irradiated nano films

Recently high-order harmonic generation (HHG) in solids has been extensively studied both experimentally and theoretically. First-principles calculations based on the time-dependent density functional theory (TDDFT) can provide realistic and reliable description for the microscopic origin of solid-state HHG. A first-principles method for realistically determining the thickness dependence of HHG emission from a laser-irradiated thin film is highly demanded but very challenging to achieve, because the microscopic HHG response is generally modulated by light propagation inside solids. From this perspective, we present first-principles study for this subject by using calculation methods combining TDDFT for electron dynamics and the Maxwell equations for light propagation. For atomically thin films, we use a coupled Maxwell-TDDFT method without a coarse graining procedure [1] . This method can take into account both effects of the light propagation and the surface electronic structure. For relatively thick films such that the surface effect is negligible, we use a Maxwell-TDDFT method with coarse graining for reducing computational cost [2] .