THz Emission Driven by Ionization Pressure in Relativistic Plasmas

The ability of terahertz (THz) waves to probe matter is attracting wide interest for applications in, e.g., remote spectroscopy. For this purpose, high field strengths with broad bandwidths are requested, which can be supplied by gas plasmas created by two-color ultrashort laser pulses. With moderate intensities ~ 10
¹⁴
W/cm
²
, intense light pulses composed of a 800-nm fundamental wavelength and its second harmonic achieve THz conversion efficiency of ~ 10
^-4
through photocurrents. In the relativistic regime, i.e., when the normalized laser vector potential a
₀
is larger than unity, plasma waves trigger a strong longitudinal field exploited for laser-wakefield electron acceleration (LWFA). The electron bunch then acts as a THz emitting source from coherent transition radiation (CTR) initiated at the plasma-vacuum interface, which may provide larger conversion efficiencies > 5 × 10
^-3
.