THz generation from relativistic plasmas driven by near- to far-infrared laser pulses

TL;DR

This paper investigates terahertz pulse generation using ultra-intense two-color laser fields in gases with near- to far-infrared wavelengths, revealing mechanisms for efficient THz emission and coherent transition radiation through particle-in-cell simulations.

Contribution

It provides a detailed analysis of THz generation mechanisms in relativistic plasmas driven by long-wavelength lasers, including analytical and numerical evaluation of emission efficiencies.

Findings

THz fields with tens of GV/m amplitude are generated in long-wavelength laser interactions.

Coherent transition radiation contributes up to 30% of total radiated energy.

Photoionization enhances plasma wakefields, causing spectral downshift and strong THz emission.

Abstract

Terahertz pulse generation by ultra-intense two-color laser fields ionizing gases with near- to far-infrared carrier wavelength is studied from particle-in-cell (PIC) simulations. For long wavelength ($10.6\ \mu$m) promoting a large ratio of electron density over critical, photoionization is shown to catastrophically enhance the plasma wakefield, causing a net downshift in the optical spectrum and exciting THz fields with tens of GV/m amplitude in the laser direction. This emission is accompanied by coherent transition radiation (CTR) of comparable amplitude due to wakefield-driven electron acceleration. We analytically evaluate the fraction of CTR energy up to 30 % of the total radiated emission including the particle self-field and numerically calibrate the efficiency of the matched blowout regime for electron densities varied over three orders of magnitude.