Phase-matched extreme-ultraviolet frequency-comb generation

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.Comment: 9 pages, 4 figure

Noncollinear enhancement cavity for record-high out-coupling efficiency of an extreme-UV frequency comb

We demonstrate a femtosecond enhancement cavity with a crossed-beam geometry for efficient generation and extraction of extreme-ultraviolet (XUV) frequency combs at a 154 MHz repetition rate. We achieve a record-high out-coupled power of 600 {\mu}W, directly usable for spectroscopy, at a wavelength of 97 nm. This corresponds to a >60% out-coupling efficiency. The XUV power scaling and generation efficiency are similar to that achieved with a single Gaussian-mode fundamental beam inside a collinear enhancement cavity. The noncollinear geometry also opens the door for the generation of isolated attosecond pulses at >100 MHz repetition rate.Comment: 13 pages, 5 figure