Scaling High-Order Harmonic Generation from Laser-Solid Interactions to Ultrahigh Intensity

TLDR

Coherent subfemtosecond x‑ray pulses generated by high‑order harmonic generation from short‑pulse, ultrahigh‑intensity lasers striking solid‑density plasmas offer a new regime for atomic physics, but the efficiency depends critically on the steepness of the plasma density gradient. At intensities above 10²¹ W cm⁻², an optimal density‑gradient scale length of c/ω₀ maximizes conversion efficiency, reduces divergence, and yields favorable power‑law scaling, while particle‑in‑cell simulations reveal that parametric instabilities and relativistic self‑phase modulation limit HOHG by degrading spectra and efficiency.

Abstract

Coherent x-ray beams with a subfemtosecond (<10(-15) s) pulse duration will enable measurements of fundamental atomic processes in a completely new regime. High-order harmonic generation (HOHG) using short pulse (<100 fs) infrared lasers focused to intensities surpassing 10(18) W cm(-2) onto a solid density plasma is a promising means of generating such short pulses. Critical to the relativistic oscillating mirror mechanism is the steepness of the plasma density gradient at the reflection point, characterized by a scale length, which can strongly influence the harmonic generation mechanism. It is shown that for intensities in excess of 10(21) W cm(-2) an optimum density ramp scale length exists that balances an increase in efficiency with a growth of parametric plasma wave instabilities. We show that for these higher intensities the optimal scale length is c/ω0, for which a variety of HOHG properties are optimized, including total conversion efficiency, HOHG divergence, and their power law scaling. Particle-in-cell simulations show striking evidence of the HOHG loss mechanism through parametric instabilities and relativistic self-phase modulation, which affect the produced spectra and conversion efficiency.