# Observation of Strong Terahertz Radiation from a Liquid Water Line

## TL;DR

This study demonstrates a novel, efficient method for generating strong terahertz radiation from a water column using femtosecond laser pulses, overcoming previous absorption limitations.

## Contribution

It reveals a new mechanism involving laser-ponderomotive-force-induced currents for terahertz generation from water, with experimental validation and potential for tabletop applications.

## Key findings

- Terahertz radiation with 0.2 MV/cm field strength generated from water.
- The mechanism relies on symmetry breaking at the water interface.
- Radiation is p-polarized regardless of laser polarization.

## Abstract

Terahertz radiation generation from liquid water has long been considered to be impossible due to strong absorption. A few very recent works reported terahertz generation from water, but the mechanism is not clear and the efficiency demands to be enhanced. We show experimentally that strong single-cycle terahertz radiation with field strength of $\rm 0.2~MV cm^{-1}$ is generated from a water line/column of $\sim 200 \mu m$ in diameter irradiated by a mJ femtosecond laser beam. This strength is 100-fold higher than that produced from air. We attribute the mechanism to the laser-ponderomotive-force-induced current with the symmetry broken around the water-column interface. This mechanism can explain our following observations: the radiation can be generated only when the laser propagation axis deviates from the column center; the deviation determines its field strength and polarity; it is always p-polarized no matter whether the laser is p- or s-polarized. This study provides a simple and efficient scheme of table-top terahertz sources based on liquid water.