Phase-matched extreme-ultraviolet frequency-comb generation

Laser-driven high-order harmonic generation1,2 provides spatially3 and temporally4 coherent tabletop sources of broadband extreme-ultraviolet (XUV) light. These sources typically operate at low repetition rates, frep ≲ 100 kHz, where phase-matched HHG is readily achieved5,6. However, many applications demand the improved counting statistics or frequency-comb precision afforded by high repetition rates, frep > 10 MHz. Unfortunately, at such high frep, phase matching is prevented by steady-state plasma accumulated in the generation volume7–11, strongly limiting the XUV average power. Here, we use high-temperature gas mixtures as the generation medium to increase the gas translational velocity, thereby reducing the steady-state plasma in the laser focus. This allows phase-matched XUV emission inside a femtosecond enhancement cavity at frep = 77 MHz, enabling a record generated power of ~ 2 mW in a single harmonic order. This power scaling opens up many demanding applications, including XUV frequency-comb spectroscopy12,13 of few-electron atoms and ions for precision tests of fundamental physical laws and constants14–20.