Polarization of THz radiation from laser generated plasma filaments

Generation of terahertz radiation by mixing fundamental and second harmonic light in laser generated plasmas is very promising for nonlinear spectroscopy and remote sensing applications. The exact origin of this very intense THz light is still not yet fully understood. Currently, there are two models being discussed in literature, namely a four-wave mixing, and a microscopic polarization model. This paper presents a detailed analysis of the dependence of the polarization of the emitted THz radiation on the input polarizations of fundamental and second-harmonic beams. Experimental results are compared with model calculations based on both four-wave rectification and the plasma current model. In these models, the THz generation generated by optical rectification in the BBO crystal and the influence of the birefringence of the BBO crystal on the fundamental beam, are taken into account.