Beam-size effects in electro-optic sampling of terahertz pulses

In electro-optic sampling of terahertz pulses with a finite diameter, the probe pulse is subject to a lensing effect. The altered diffraction leads to an additional phase shift Deltaeta in the far field compared to the case of a plane wave. It is demonstrated that Deltaeta is in the same order of magnitude as the direct electro-optic phase shift at the exit of the detection crystal and thus also gives a significant contribution to the measured amplitude.