Harmonic Generation from Relativistic Plasma Surfaces in Ultra-Steep Plasma Density Gradients

TL;DR

This study experimentally investigates harmonic generation from relativistic plasma surfaces with ultra-steep density gradients, revealing optimal conditions for high harmonic efficiency and quantifying photon yields in the extreme ultraviolet range.

Contribution

It demonstrates the dependence of harmonic generation efficiency on plasma density scale-lengths and provides calibrated measurements of photon yields for ultra-steep gradients.

Findings

Efficient high-order harmonic generation requires steep plasma density gradients.

Harmonic efficiency decreases as the plasma density scale-length approaches zero.

Photon yields range from 10^{-4} to 10^{-6} of laser energy for 20-40 eV photons.

Abstract

Harmonic generation in the limit of ultra-steep density gradients is studied experimentally. Observations demonstrate that while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale-lengths ($L_p/\lambda < 1$) the absolute efficiency of the harmonics declines for the steepest plasma density scale-length $L_p \to 0$, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the Relativistic Oscillating Mirror (ROM) was estimated to be in the range of 10^{-4} - 10^{-6} of the laser pulse energy for photon energies ranging from 20-40 eV, with the best results being obtained for an intermediate density scale-length.