An analysis of the polarization of terahertz (THz) radiation from a laser-induced plasma source is presented. THz emission is achieved by mixing a laser pulse with its second harmonic after focusing through a (β-BBO) crystal. Numerical calculations, based on the nonlinear four-wave mixing model and the microscopic polarization model, are compared with experimental results. The main focus lies on the study of the dependence of THz polarization on the polarization and relative phase of the incident fundamental and second-harmonic pulses. We show that the modulation of the fundamental pulse by the BBO crystal has to be taken into account in order to describe experimental observations. By including the finite extension of the plasma and considering cross- and self-phase modulation of the two-color pump pulse, we are able to explain the observed ellipticity of the THz pulse as well as the orientation of the polarization axis.
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