ABSTRACT
This chapter describes the method of extending technologies of terahertz (THz) air photonics to THz liquid photonics, using laser-induced micro-plasma with flowing liquid targets. Matter exists in distinct states. Among them, solids, gases, and plasmas have been used as sources of THz waves for decades. Exploring new THz sources could provide an alternative perspective in understanding fundamental physics. However, the current use of liquids, especially liquid water, as THz wave emitters was extremely limited until very recently. Broadband terahertz wave generation from liquid was first demonstrated in 2017. Since then, it has been widely studied with different liquid targets under various excitation geometries. The recent progress and challenges are summarized in this book chapter. It is reasonable to expect that liquids should have unique properties if they could be harnessed as THz sources.
