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.