Tuning High-Harmonic Generation by Controlled Deposition of Ultrathin Ionic Layers on Metal Surfaces

TL;DR

This study demonstrates that the harmonic spectrum in high harmonic generation can be precisely tuned by depositing ultrathin insulator layers on metal surfaces, with the cutoff increasing linearly with layer thickness, offering a new control method independent of laser parameters.

Contribution

The paper introduces a novel approach to control high harmonic generation by varying the thickness of ultrathin ionic layers on metal surfaces, supported by numerical simulations of NaCl/Cu(111) systems.

Findings

Harmonic cutoff increases linearly with the number of monolayers.

Control over HHG spectrum surpasses that achieved by changing laser intensity.

Electronic 'friction' decreases as the system transitions from discrete to continuous energy spectra.

Abstract

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, not the laser properties. This is shown from numerical solutions of the time-dependent Schr\"odinger equation for $n$ML NaCl/Cu(111) systems ($n=1-50$) based on realistic potentials available in the literature. 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. Furthermore, the degree of control that can be achieved in this way is much higher than by varying the laser intensity. The origin of this behavior is the reduction of electronic "friction" when moving from the essentially discrete energy spectrum associated with a few-ML system to the continuous energy spectrum (bands) inherent to an extended periodic system.