Research Progress in Terahertz Liquid Photonics (Invited)

The use of liquids as terahertz (THz) wave emitters and detectors has been historically avoided due to the high absorption of polar liquids in the THz range, especially liquid water. This hinders the development of THz liquid photonics. Compared with other matter states, liquids exhibit numerous unique properties. In particular, liquids have a material density comparable to that of solids, meaning that the number of molecules interacting with laser pulses is three times higher than that of gas. In contrast to solids, liquid fluidity allows each laser pulse to interact with a fresh target area. Therefore, the material damage threshold is not an issue even with high repetition rate laser pulses. This makes liquids very promising candidates for studying high-energy-density plasma and ultrafast dynamics of ionized particles in laser-matter interaction. THz liquid photonics is an emerging topic, offering an alternative for researchers to obtain THz emission from liquid material. This interdisciplinary and transformative topic will enable new science and advance numerous THz wave sensing and spectroscopy technologies that significantly impact THz technology, including nextgeneration liquid source, device, and system development.