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We characterize a terahertz (THz) source based on plasma in liquid gallium. The dependence of the emitted THz pulse energy on second-order phase, pump pulse energy, and polarization of the short laser pulse is demonstrated. Our study suggests that the THz emission mechanism is due to the ponderomotive force and is aided by a direct-field driven term. The proposed source and accompanying generation mechanism are studied under a non-relativistic regime ( $10^{15}<<\#comment/>\mathrm{I}<<\#comment/>10^{18}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$) for forward directed THz under a single pump excitation scheme. 

Terahertz wave emission from liquids excited by intense laser pulses not only reflects the details in laser–matter interaction but also offers bright terahertz wave sources. Flowing liquid targets possess the advantage of providing a fresh area for each laser pulse. To demonstrate a debris-free target under laser excitation, we investigate the use of liquid nitrogen as a target. By creating a flowing liquid nitrogen line in an ambient environment, we successfully observe broadband terahertz wave emission under short pulse excitation. Our cryogenic line is able to sustain the excitation of a high-repetition-rate (1 kHz) laser. The terahertz peak field emitted from liquid nitrogen is comparable to that from liquid water, yet a broader bandwidth is observed. This demonstration prompts opportunities in choosing potential materials for studying terahertz wave generation processes and in understanding laser-induced ionization of different liquids.