Ultra-broadband, intense, coherent terahertz (THz) radiation can be generated, detected, and manipulated using laser-induced gas or liquid plasma as both the THz wave transmitter and detector, with a frequency coverage spanning across and beyond the whole “THz gap.” Such a research topic is termed “plasma-based THz wave photonics in gas and liquid phases.” In this paper, we review the most important experimental and theoretical works of the topic in the non-relativistic region with pump laser intensity below 1018W/cm2.

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ～50 % of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ / pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

Terahertz (THz) wave generation from laser-induced air plasma generally requires a short temporal laser pulse. In contrast, it was observed that THz radiation from ionized liquid water prefers a longer pulse, wherein the mechanism remains unclear. We attribute the preference for longer pulse duration to the process of ionization and plasma formation in water, which is supported by a numerical simulation result showing that the highest electron density is achieved with a subpicosecond pulse. The explanation is further verified by the coincidence of our experimental result and simulation when the thickness of the water is varied. Other liquids are also tested to assure the preference for such a pulse is not exclusive to water.