Terahertz radiation from shockwave front irradiated with femtosecond optical pulses

Femtosecond double optical pulse experiments with nanosecond time delay were conducted in air. Terahertz pulses were generated from laser plasmas produced by the irradiation with the main pulses on shockwave excited by the pre-pulse illumination. Intensity of the terahertz pulses depends on the time difference Delta t and relative position Delta y between the pre- and main pulses. We found a characteristic dependence of the intensity of the terahertz pulses on Delta y at a longer time delay, Delta t = 14.7 ns, which has two maxima while only one maximum was observed when delay times were at and shorter than 9.7 ns. The results are discussed taking into account air density distribution of the shockwave. The terahertz emission spectroscopy shown here can be a useful technique for investigating density structure and its time variation of shockwave with high temporal resolution.