"The electrons ionized from water molecules are regarded as quasi-free electrons, and therefore the THz wave generation process in water resembles this process in air. Phenomenologically, the transient photocurrent model can be used to explain the generation process in water and would predict the modulation of THz fields generated from a water film as well, which is experimentally confirmed, as shown in Fig. 3. Specifically, Fig. 3(a) shows that the polarity of the THz electric field is completely flipped over by changing the relative phase by pi."

The terahertz radiation has a maximum when the relative phase is zero, but why does the author say that this comes from the ionization current model?

Isn't this contradicting the ionization current model mentioned by K.Y.Kim? The terahertz radiation has a maximum value when the relative phase is pi/2 in the ionization current model proposed by K.Y. Kim.

https://aip.scitation.org/doi/pdf/10.1063/1.5064644?class=pdf