# Terahertz-induced cascaded interactions between spectra offset by large frequencies

## TL;DR

This paper investigates how terahertz-driven cascaded nonlinear interactions can couple optical spectra with large frequency differences, enabling efficient terahertz generation and insights into damage mitigation.

## Contribution

It introduces a model for cascaded second-order nonlinear processes driven by terahertz radiation with large spectral offsets, predicting high-efficiency terahertz generation.

## Key findings

- Percent-level terahertz conversion efficiency predicted.
- Millijoule-level terahertz pulse energies achievable.
- Strategies for reducing laser-induced damage outlined.

## Abstract

We explore the dynamics of a system where input spectra in the optical domain with very disparate center frequencies are strongly coupled via highly phase-matched, cascaded second-order nonlinear processes driven by terahertz radiation. The only requirement is that one of the input spectra contain sufficient bandwidth to generate the phase-matched terahertz-frequency driver. The frequency separation between the input spectra can be more than ten times larger than the phase-matched terahertz frequency. A practical application of such a system where the cascading of a narrowband pump line centered at 1064 nm induced by a group of weaker seed lines centered about 1030 nm and separated by the phase-matched terahertz frequency is introduced. This approach is predicted to generate terahertz radiation with percent-level conversion efficiencies and millijoule-level pulse energies in cryogenically-cooled periodically poled lithium niobate. A model that solves for the nonlinear coupled interaction of terahertz and optical waves is employed. The calculations account for second and third-order nonlinearities, dispersion in the optical and terahertz domains as well as terahertz absorption. Ramifications of pulse formats on laser-induced damage are estimated by tracking the generated free-electron density. Strategies to mitigate laser-induced damage are also outlined.