Terahertz-induced second harmonic radiation from transient electron behavior in water

Laser-induced liquid plasma is expected to be an efficient medium for a new generation of terahertz (THz) devices. However, the mechanism of interaction between liquid plasma and THz waves is currently unclear. Here, we observed two phenomena related to THz wave interaction with water plasma: THz-induced second harmonic (TISH) radiation in water plasma and THz absorption by water plasma. We established an internal relationship between these two macroscopic phenomena by utilizing a photocurrent model and the Drude model. Using liquid water and air as examples, a scheme is proposed to analyze the photonic characteristics of plasma in different physical states. This study offers an experimental basis for predicting the TISH phenomenon across various substances and states and also provides the theoretical support for the practical implementation of liquid-based detection of THz waves.