Tuning high-harmonic generation by controlled deposition of ultrathin ionic layers on metal surfaces

High-harmonic generation (HHG) from semiconductors and insulators has become a very active area of research due to its great potential for developing compact HHG devices. Here we show, that by growing monolayers (ML) of insulators on single-crystal metal surfaces, one can tune the harmonic spectrum by just varying the thickness of the ultrathin layer, rather than the laser properties. This is shown from numerical solutions of the time-dependent Schrodinger equation for Cu(111)/n-ML NaCl systems (n = 1-50) based on realistic potentials. Remarkably, the harmonic cutoff increases linearly with n and as much as an order of magnitude when going from n = 1 to 30, while keeping the laser intensity low and the wavelength in the near-infrared range. The origin of this behavior is twofold: the initial localization of electrons in a Cu-surface state and the reduction of electronic "friction" when moving from the essentially discrete energy spectrum associated with a few-ML system to the continuous spectrum (bands) inherent in extended periodic systems. Our findings are valid for both few-and multicycle IR pulses and wavelengths similar to 1-2 mu m.