Hybrid Stimulated Raman Scattering -- Two Plasmon Decay Instability and 3/2 Harmonic in Steep-Gradient Femtosecond Plasmas

TL;DR

This paper investigates the dominant role of hybrid Stimulated Raman Scattering and Two Plasmon Decay instabilities in plasma wave excitation and energy absorption during intense femtosecond laser interactions with steep-gradient plasmas, revealing new harmonic generation mechanisms.

Contribution

It demonstrates the dominance of hybrid Stimulated Raman Scattering and Two Plasmon Decay instabilities in plasma excitation and introduces a new mechanism for 3/2 harmonic generation involving plasma wave high harmonics.

Findings

Plasmons are excited as two wave packets near the surface with wide spectral range.

Phase matching for 3/2 harmonic generation is fulfilled with a new mechanism.

Experimental observation of an additional 3/2 harmonic beam supports the new mechanism.

Abstract

The hybrid Stimulated Raman Scattering - Two Plasmon Decay instability had shown to play the dominant role for plasma waves excitation and energy absorption at interaction of very intense femtosecond obliquely incident $p$-polarized laser pulse (intensity $10^{17}-5\cdot10^{19}$ W/cm$^2$) with a steep gradient ($L\sim \lambda$) plasma near the quarter critical surface, i.e. within the typical for modern experiments conditions. The plasmons are excited as two wave packets confined near this surface with very wide $\approx \omega_0/c$ spatial spectra along its normal. Hence, phase matching conditions for 3/2 harmonic generation fulfill immediately and include new mechanism coming from high harmonics of plasma waves. The latter mechanism have been proved experimentally observing an additional 3/2 harmonic beam.