title: Bright high-repetition-rate source of narrowband extreme-ultraviolet harmonics beyond 22 eV.
abstract: 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 ≈3 × 10(13) s(-1) is generated at 22.3 eV, with 5 × 10(-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.
authors.affiliation: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, MS 2-354, 1 Cyclotron Road, Berkeley, California 94720, USA.
authors.forename: He
authors.identifier: 0000000187326545
authors.initials: H
authors.lastname: Wang
authors.affiliation: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, MS 2-354, 1 Cyclotron Road, Berkeley, California 94720, USA.
authors.forename: Yiming
authors.initials: Y
authors.lastname: Xu
authors.affiliation: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, MS 2-354, 1 Cyclotron Road, Berkeley, California 94720, USA.
authors.forename: Stefan
authors.initials: S
authors.lastname: Ulonska
authors.affiliation: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, MS 2-354, 1 Cyclotron Road, Berkeley, California 94720, USA.
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Robinson
authors.affiliation: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, MS 2-354, 1 Cyclotron Road, Berkeley, California 94720, USA.
authors.forename: Predrag
authors.initials: P
authors.lastname: Ranitovic
authors.affiliation: Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, MS 2-354, 1 Cyclotron Road, Berkeley, California 94720, USA.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Kaindl