Temporal THz signal measured using a 200-μm-thick GaP (a) and a 20-μm-thick ZnTe crystal (b), and their corresponding FFT amplitude spectra in logarithm (c) and linear scales (d). The spectral dips at 5.3 THz and 8 THz are caused by ZnTe detector (phonon mode@5.3 THz) and GaAs crystal (phonon mode@8 THz), respectively.