Bright high-repetition-rate source of narrowband extreme-ultraviolet harmonics beyond 22 eV

Novel table-top sources of extreme-ultraviolet light based on high-harmonic generation yield unique insight into the fundamental properties of molecules, nanomaterials, or correlated solids, and enable advanced applications in imaging or metrology. Extending high-harmonic generation to high repetition rates portends great experimental benefits, yet efficient extreme-ultraviolet conversion of correspondingly weak driving pulses is challenging. Here, we demonstrate a highly efficient source of femtosecond extreme-ultraviolet pulses at 50-kHz repetition rate, utilizing the ultraviolet second-harmonic focused tightly into Kr gas. In this cascaded scheme, a photon flux beyond ~3e13 per second is generated at 22.3 eV, with 5e-5 conversion efficiency that surpasses similar harmonics directly driven by the fundamental by two orders of magnitude. The enhancement arises from both wavelength scaling of the atomic dipole and improved spatio-temporal phase-matching, confirmed by simulations. Spectral isolation of a single 72-meV wide harmonic renders this bright, 50-kHz extreme-ultraviolet source a powerful tool for ultrafast photoemission, nanoscale imaging and other applications.