THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

We present a study of the competition between tera-hertz (THz) generation by optical rectification in <110> ZnTe crystals, two-photon absorption, secondharmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects fromthe sub-wavelength THz source.