High-power femtosecond molecular broadening and the effects of ro-vibrational coupling

TL;DR

This paper demonstrates high-power femtosecond spectral broadening in molecular gases, especially nitrogen, at unprecedented power levels, enabling advanced ultrafast applications and providing new insights into molecular Raman processes.

Contribution

It introduces a method for achieving ten-fold spectral broadening at 250W in nitrogen, surpassing previous limits, and offers a new understanding of molecular ro-vibrational effects on spectral broadening.

Findings

Spectral broadening in nitrogen at 250W power levels.

Pulse compression from 1.3ps to 120fs achieved.

High harmonic generation demonstrated at 200kHz.

Abstract

Scaling spectral broadening to higher pulse energies and average powers, respectively, is a critical step in ultrafast science, especially for narrowband Yb based solid state lasers which become the new state of the art. Despite their high nonlinearity, molecular gases as the broadening medium inside hollow core fibers have been limited to 25 W, at best. We demonstrate spectral broadening in nitrogen at ten-fold average powers up to 250W with repetition rates from 25 to 200kHz. The observed ten-fold spectral broadening is stronger compared to the more expensive krypton gas and enables pulse compression from 1.3ps to 120fs. We identified an intuitive explanation for the observed average power scaling based on the density of molecular ro vibrational states of Raman active molecules. To verify this ansatz, spectral broadening limitations in O2 and N2O are experimentally measured and agree well. On these grounds we propose a new perspective on the role, suitability, and limits of stimulated Raman scattering at high average and peak powers. Finally, high harmonic generation is demonstrated at 200 kHz.