Phase-matched extreme-ultraviolet frequency-comb generation

TL;DR

This paper demonstrates phase-matched extreme-ultraviolet frequency comb generation at high repetition rates using hot gas mixtures, achieving record power levels suitable for precision spectroscopy and fundamental physics tests.

Contribution

It introduces a novel method of using high-temperature gas mixtures to enable phase matching at high repetition rates in XUV generation, overcoming previous power limitations.

Findings

Achieved phase-matched XUV emission at 77 MHz

Generated record power of ~2 mW in a single harmonic

Enabled applications in high-precision XUV spectroscopy

Abstract

Laser-driven high-order harmonic generation (HHG) provides tabletop sources of broadband extreme-ultraviolet (XUV) light with excellent spatial and temporal coherence. These sources are typically operated at low repetition rates, $f_{rep}\lesssim$100 kHz, where phase-matched frequency conversion into the XUV is readily achieved. However, there are many applications that demand the improved counting statistics or frequency-comb precision afforded by operation at high repetition rates, $f_{rep}$ > 10 MHz. Unfortunately, at such high $f_{rep}$, phase matching is prevented by the accumulated steady-state plasma in the generation volume, setting stringent limitations on the XUV average power. Here, we use gas mixtures at high temperatures as the generation medium to increase the translational velocity of the gas, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at a repetition rate of 77 MHz, enabling a record generated power of $\sim$2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy of few-electron atoms and ions for precision tests of fundamental physical laws and constants.