Sequential terahertz pulse generation by photoionization and coherent transition radiation in underdense relativistic plasmas

TL;DR

This paper investigates how ultrashort two-color laser pulses interacting with underdense helium gases generate terahertz radiation through sequential mechanisms, with simulations and analytical models revealing dominant processes and high field strengths.

Contribution

It introduces a comprehensive analysis of sequential THz generation mechanisms in underdense plasmas, supported by simulations and analytical models, highlighting the dominance of coherent transition radiation at high densities.

Findings

CTR dominates THz emission at densities > 10^{17} cm^{-3}

THz bursts reach field strengths of 100 GV/m and energies over 1 mJ

Analytical models accurately reproduce simulation results

Abstract

Terahertz (THz) emission by two-color, ultrashort optical pulses interacting with underdense helium gases at ultrahigh intensities ($> 10^{19}\,\mathrm{W/cm}^2$) is investigated by means of 3D particle-in-cell simulations. The THz field is shown to be produced by two mechanisms occurring sequentially, namely, photoionization-induced radiation (PIR) by the two-color pulse and coherent transition radiation (CTR) by the wakefield-accelerated electrons escaping the plasma. For plasmas of atomic densities $> 10^{17}\,\mathrm{cm}^{-3}$, CTR proves to be the dominant process, providing THz bursts with field strength as high as $100\,\mathrm{GV/m}$ and energy in excess of $1\,\mathrm{mJ}$. Analytical models are developed for both the PIR and CTR processes, which correctly reproduce the simulation data.