Terahertz Nonlinear Spectroscopy of Liquid Water

Recently, the classical Z-scan technique is considered feasible for the nonlinear measurement of liquid water in the low-frequency terahertz (THz) band. However, this scheme ignores the nonlinear spectral variation of broadband THz pulses, and its investigation of liquid water in a higher THz band (>1 THz) remains challenging until now. Here, we use Z-scan technology to directly measure the nonlinear absorption coefficient and nonlinear refractive index of liquid water excited at a higher THz range of 0.1-3 THz. Combined with THz time-domain spectroscopy technology, the spectral transmission characteristics caused by nonlinear effects in the Z-scan process are studied for the first time. The potential influence of the saturation absorption effect and self-phase modulation on the transmitted THz beam in liquid water are analyzed theoretically, and the origin of the nonlinear spectral distortion of the THz pulse is further revealed. This work offers a new perspective for observing the nonlinear transmission of THz pulses in liquid water and provides an experimental basis for more accurate analysis of the nonlinear characteristics of water-based THz devices in the future.