# Flat liquid jet as a highly efficient source of terahertz radiation

## TL;DR

This paper demonstrates that flat liquid jets can serve as highly efficient, damage-resistant sources of terahertz radiation when excited with sub-picosecond optical pulses, with efficiency comparable to existing methods.

## Contribution

It introduces a systematic study of terahertz generation in flat liquid jets, revealing optimal conditions and efficiency, advancing liquid-based terahertz source development.

## Key findings

- Terahertz energy scales quasi-quadratically with pulse energy.
- Optimal pump pulse duration depends on jet thickness.
- Achieved optical-to-terahertz efficiency exceeds 0.05%.

## Abstract

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.