Laser-triggered terahertz emission from near-critical-density targets

Femtosecond laser pulse propagation in a relativistic self-trapping (RST) regime in a near-critical density plasma makes it possible to maximize the total charge of the accelerating electrons and laser-to-electrons conversion rate, that can be used to provide a large amount of the terahertz range coherent transition radiation. The three-dimensional particle-in-cell simulations demonstrate how such transition radiation generates when electrons escape into vacuum either from the low-density target itself, or after passing through a thin foil located at the target end. The advantage of the RST regime for the generation of terahertz pulses is clearly demonstrated as compared to laser irradiation of such a standard target as a foil with preplasma on its front side. The simulation performed has shown that for the optimized laser-target matching a 2-J femtosecond laser pulse is able to produce quasiunipolar terahertz pulses with energy exceeding 100 mJ.