Polarization analysis of terahertz radiation generated by four-wave mixing in air.

We examine the generation of terahertz (THz) by optical rectification of a fundamental infrared beam with its first harmonic in ionized air. From polarization measurements we identify an important, yet so far unreported (to our knowledge), cross term (chi(3)(xyxy)) + chi(3)(xyyx)) of the nonlinear susceptibility tensor. Omission of this term leads to an error by a factor of 10(6) of the THz intensity obtained with certain polarization configurations.