Mapping the power-law decay of high-harmonic spectra from laser-plasma interactions

TL;DR

This paper investigates how the spectral decay in high-harmonic generation from laser-plasma interactions varies with different experimental parameters, challenging the notion of a universal power-law exponent.

Contribution

It demonstrates through simulations that the decay exponent is a continuous function of interaction parameters, not a fixed universal value.

Findings

Decay exponent varies with laser and plasma parameters

No single universal power-law exponent exists

Optimization opportunities identified for harmonic generation efficiency

Abstract

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 the spectral components of the reflected signal 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. At relativistic intensities, a spectrum exhibiting a power-law decay with an exponent of 8/3 or 4/3 is often stated. Here, we analyse the dependence of this exponent on interaction parameters including the angle of incidence, the carrier envelope phase, intensity of the laser and the pre-plasma length, and discuss opportunities for optimization. Our simulations show that, rather than there being one universal exponent, the spectral decay is a continuous function of the laser-plasma interaction parameters.