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))(xyxy)) 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. (C) 2008 Optical Society of America.