Finite pump-beam-size effects in optical pump-terahertz probe spectroscopy

Finite pump-beam size in optical pump-terahertz (THz) probe experiments introduces distortions in both the signal amplitude and the spectral form. We show both experimentally and analytically that the pump beam must be 3-4 times larger than the THz probe beam for a one-photon excitation process to achieve <10% spectral distortion. Our experiments use thin samples of single-crystal ZnTe or CdTe. Our model treats the pump-induced change in the THz probe field as radiation from a photoinduced current driven by the THz field. Interpretation of nonideal measurements and relaxation of the requirement on the pump-beam size by introduction of a metallic aperture are also discussed.