Backward terahertz radiation from intense laser-solid interactions

We report a systematic study on backward terahertz (THz) radiation generation from laser-solid interactions by changing a variety of laser/plasma parameters. We demonstrate a high-energy (with an energy flux density reaching 80 μJ/sr), broadband (10 THz) plasma-based radiation source. The radiation energy is mainly distributed either in the10 THz or3 THz regions. A radial surface current formed by the lateral transport of low-energy electrons (LEE) is believed to be responsible for the radiation in the high-THz region (10 THz), while high-energy surface fast electrons (SFE) accelerated along the target surface mainly contribute to lower frequency (3 THz) radiation. The unifying explanation could be applied to backward THz radiation generation from solid targets with presence of relative small preplasmas.