Hot-electron refluxing in thin foils irradiated by ultraintense laser pulses and highly efficient terahertz radiation

Radially polarized intense terahertz (THz) radiation is measured experimentally behind a thin foil irradiated by ultrashort relativistic intense laser pulse of ultrahigh contrast. As the target thickness is reduced from 30 to 2 μm, the energy of the THz emission increases dramatically, which even reaches up to 10.5 mJ per pulse, corresponding to energy conversion efficiency of 1.7%. This efficient THz emission is attributed to bremsstrahlung radiation of the energetic electrons rapidly refluxing between the target front and back electrostatic sheaths fields created during the laser-target interaction. Analytical modeling and particle-in-cell simulation confirms this interpretation.