A noncollinear enhancement cavity for record-high out-coupling efficiency of 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 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 generation of isolated attosecond pulses at >100 MHz repetition rate.