Terahertz emission profile from laser-induced air plasma

We report the characterization of the emission profile of terahertz waves generated by four-wave mixing in the presence of laser-induced plasma. Highly directional terahertz waves with a divergence angle smaller than 10 degrees are measured in the presence of long plasmas (> 10 mm). Frequency-dependent interference structures in the angular distribution of the radiation are observed under tighter focusing conditions and are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz generation from four-wave mixing in air plasmas is a promising source for spectroscopy and imaging in the terahertz range.