Engineering the Relativistic High Harmonic Generation from Ultrathin Foils

The interaction of intense laser pulses with plasma underlies a suite of cutting-edge applications—including high-order harmonic generation (HHG) for attosecond pulse production. In this study, we employ two-dimensional particle-in-cell (PIC) simulations to examine how ultrathin foils interact with high-intensity laser pulses to drive relativistic HHG.

Ultrathin foils—whose thickness is much less than the driving laser wavelength—are particularly advantageous as they enable efficient extraction of both transmitted and reflected harmonics, maximizing conversion efficiency when optimized at nanometer-scale thicknesses. Two-dimensional materials, due to their atomic-scale thickness and mechanical flexibility, serve as ideal candidates for these ultrathin targets. Through strain engineering, they can adopt a range of nanoscale geometries, potentially emulating the curved plasma mirror effect, which has been shown to dramatically enhance harmonic focus and intensity via coherent focusing mechanisms. We propose combining ultrathin 2D materials with pre-designed curvature to investigate whether strain-engineered surface geometry can significantly boost HHG intensity and better control beam divergence—without relying on laser-induced curvature alone. Using 2D PIC simulations, we show how variations in target shape influence HHG efficiency and beam quality, aiming to identify the strategies that yield optimal attosecond output.