Terahertz Pump-Terahertz Probe Spectroscopy Revealing Pockels and Kerr Effects in ZnTe

Terahertz (THz) pump–THz probe spectroscopy has emerged as a powerful platform for investigating ultrafast nonlinear light–matter interactions. However, the physical origin of recently reported THz signal enhancement remains controversial because contributions from THz generation and electro-optic detection are often coupled. Here, we use a Fe₄GeTe₂ spintronic THz emitter together with a ZnTe electro-optic detector to demonstrate that with fixed THz pump polarity, reversing the magnetic-field direction of the Fe₄GeTe₂ emitter does not change the signal enhancement or induce suppression, indicating that the modulation originates from the nonlinear electro-optic response of the ZnTe detector rather than the THz emitter. Furthermore, controlling the polarity of the THz pump field reveals a pronounced asymmetric response: a positive THz field induces signal enhancement, whereas a negative field leads to suppression, revealing strong polarity-dependent THz pump–THz probe modulation. This behavior cannot be solely attributed to carrier redistribution in the conduction band and is consistently described by a model incorporating both linear Pockels and nonlinear Kerr effects in ZnTe. This polarity-dependent reversal of the linear Pockels contribution accounts for the observed polarity-dependent THz-field-induced absorption modulation. These results clarify the physical origin of THz pump–THz probe modulation, establish detection-induced nonlinearity as the dominant mechanism governing the measured response, and highlight the importance of treating electro-optic detection as an active nonlinear element in THz pump–THz probe spectroscopy.