Intense broadband terahertz pulses produced in relativistic laser-plasma interaction in the mid-IR

Broadband, ultrashort THz pulses are of increasing interest for applications. We present results from particle-in-cell simulations demonstrating that mid-infrared laser-driven near-critical density plasmas can have extremely high THz conversion efficiency. Specifically, a tightly focused 25 mJ, 80 fs λ=3.9μm pulse in a fully ionized hydrogen gas jet plasma gives up to 8% conversion efficiency, generating a ~2 mJ 1-30 THz pulse. These simulations show that the frequency conversion is mainly due to downshifting from co-propagation of the mid-IR pulse and its self-generated plasma wake bubble, with a smaller contribution from coherent transition radiation by laser wakefield-accelerated electrons exiting the gas jet. The maximum conversion efficiency is strongly correlated with a narrow range of peak plasma density and propagation length that avoid bubble collapse.