  Polar liquids are strong absorbers of electromagnetic waves in the terahertz
range, therefore, historically such liquids have not been considered as good
candidates for terahertz sources. However, flowing liquid medium has explicit
advantages, such as a higher damage threshold compared to solid-state sources
and more efficient ionization process compared to gases. Here we report
systematic study of efficient generation of terahertz radiation in flat liquid
jets under sub-picosecond single-color optical excitation. We demonstrate how
medium parameters such as molecular density, ionization energy and linear
absorption contribute to the terahertz emission from the flat liquid jets. Our
simulation and experimental measurements reveal that the terahertz energy has
quasi-quadratic dependence on the optical excitation pulse energy. Moreover,
the optimal pump pulse duration, which depends on the thickness of the jet is
theoretically predicted and experimentally confirmed. The obtained
optical-to-terahertz energy conversion efficiency is more than 0.05%. It is
comparable to the commonly used optical rectification in most of
electro-optical crystals and two-color air filamentation. These results,
significantly advancing prior research, can be successfully applied to create a
new alternative source of terahertz radiation.
