Fig. 1. (a) Diagram of the experimental system. TPs, THz polarizers; PM, parabolic mirror; EOS, electro-optical sampling. (b) The spectrums of THz pulses propagating through the water film and nitrogen gas (remove the water film) without an optical-pump, respectively.

Fig. 2. (a) THz time-domain waveforms in liquid water without an optical pump (black line) and under the maximal absorption caused by the formed plasma (red line); the corresponding results for nitrogen gas are shown in the inset. (b) Transient evolution curve of THz wave absorption by plasma in water (and nitrogen gas) under a subpicosecond observation window with the pump energy of 90μJ/pulse, and the corresponding fitted results for water are shown with the orange line. The inset shows the experimental data and fitted results of the transient evolution process of THz wave absorption by plasma in water under the observation window of several hundred picoseconds.

Fig. 3. Experimental (orange solid line) and simulated (orange dotted line) frequency-domain characteristics of THz wave absorption under the maximal modulation by plasma in water. The black solid line and the black dotted line show the spectra of THz pulses with the 90-μJ pump pulse (under the maximal absorption) and without pump pulse, respectively.

Fig. 4. (a) Transient evolution curves of THz wave absorption by plasma in water induced by different pump pulse energies. The inset shows the corresponding normalized results from 0 ps. (b) The black points indicate the peak quasifree electron density with different pump pulse energies. The black line is the linear fitting results. The orange points show the relationship between the solvation ratio and the pump pulse energy at the equilibrium state.