Localized terahertz generation via optical rectification in ZnTe

We report the measurement of terahertz (THz) radiation, generated via optical rectification in a 20 μm thick ZnTe crystal, as a function of the size of optical excitation. The result shows that, before the onset of significant higher-order nonlinear processes, the THz emission obtained with a fixed excitation power is largely size independent for excitation sizes smaller than the THz wavelength. This experimental finding is well described by a theoretical model including the generation of THz radiation through optical rectification from a subwavelength source and its propagation into the far field. The characteristic size dependence of the radiation from a subwavelength THz source is advantageous for use in apertureless near-field microscopy.