Strong Terahertz Radiation from a Liquid-Water Line

Terahertz generation from liquid, in particular water, was long considered impossible, because of strong absorption. While terahertz radiation from liquid water was finally observed in 2017, the mechanism has remained unclear, and the efficiency low. Here experiments show that when a column of flowing water is used, the efficiency can be enhanced by three orders of magnitude---as high as for a typical two-color air plasma source, yet using a single-color laser pump here. The authors explain the mechanism with a model of current induced by laser ponderomotive force, which is supported by particle-in-cell simulations.