Exponent of the spectral power–law from High–order Harmonic generation

Visible or near infra-red light can be manipulated to produce bursts of coherent extreme ultraviolet (XUV) or X-rays via the relativistic high-order harmonic generation process, when a laser irradiates a solid plasma target. The intensity of spectral components decays with increase of harmonic order and the efficiency of this non-linear process largely hinges on how prompt this decay is. This is governed by the conditions of the laser-plasma interaction, for which various models have been proposed. Using 1D PIC simulations, we analyse the trends for the reemitted spectral decay for different laser-plasma interaction parameters, particularly the pre-plasma scale length, the incidence angle and the carrier envelope phase, and discuss the optimization opportunities. Our simulations show that, rather than there being one universal exponent, the spectral decay is a continuous function of laser-plasma interaction parameters [1]. Such thorough parameter-scans of an individual interaction scenario with additional machine-learning-based diagnostics [2] can advance plasma-HHG as a rich radiation source.