Broadband THz emission from gas plasmas induced by femtosecond optical pulses:: From fundamentals to applications

We review the generation of broadband THz radiation from femtosecond photo-induced gas plasmas, with an emphasis on the highly efficient AC-bias case where the plasma is generated and driven by a superposition of fundamental and second-harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi-quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four-wave mixing model. We also employ these models to the case of few-cycle pulses, where the observed THz emission is related directly to the carrier-envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.