Portraying polarization state of terahertz pulse generated by a two-color laser field in air.

By using an electro-optic sampling technique, the instantaneous field vector of the terahertz (THz) pulse generated by mixing a near-IR femtosecond pulse and its second harmonic in air has been studied. The polarization of the investigated THz pulse is found to be predominantly linear and orthogonal to that of the second harmonic, revealing the major role of chi((3))(xyxy) during the four-wave mixing process.