title: Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.
abstract: High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching--the constructive addition of x-ray waves from a large number of atoms--favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth-limited pulse trains of ~100 attoseconds.
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Dimitar
authors.initials: D
authors.lastname: Popmintchev
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA. Grupo de Investigación en Óptica Extrema, Universidad de Salamanca, E-37008 Salamanca, Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Hernández-García
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Franklin
authors.initials: F
authors.lastname: Dollar
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Mancuso
authors.affiliation: Centro de Laseres Pulsados, E-37008 Salamanca, Spain.
authors.forename: Jose A
authors.initials: JA
authors.lastname: Pérez-Hernández
authors.affiliation: Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan.
authors.forename: Ming-Chang
authors.initials: MC
authors.lastname: Chen
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Amelia
authors.initials: A
authors.lastname: Hankla
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Xiaohui
authors.initials: X
authors.lastname: Gao
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Bonggu
authors.initials: B
authors.lastname: Shim
authors.affiliation: School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA.
authors.forename: Alexander L
authors.initials: AL
authors.lastname: Gaeta
authors.affiliation: Department of Chemistry, Temple University, Philadelphia, PA 19122, USA.
authors.forename: Maryam
authors.initials: M
authors.lastname: Tarazkar
authors.affiliation: Department of Physics, Temple University, Philadelphia, PA 19122, USA.
authors.forename: Dmitri A
authors.initials: DA
authors.lastname: Romanov
authors.affiliation: Department of Chemistry, Temple University, Philadelphia, PA 19122, USA.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Levis
authors.affiliation: Physics Division, Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
authors.forename: Jim A
authors.initials: JA
authors.lastname: Gaffney
authors.affiliation: Physics Division, Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Foord
authors.affiliation: Physics Division, Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Libby
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Jaron-Becker
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Andreas
authors.initials: A
authors.lastname: Becker
authors.affiliation: Grupo de Investigación en Óptica Extrema, Universidad de Salamanca, E-37008 Salamanca, Spain.
authors.forename: Luis
authors.initials: L
authors.lastname: Plaja
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Murnane
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA.
authors.forename: Henry C
authors.initials: HC
authors.lastname: Kapteyn
authors.affiliation: JILA, University of Colorado, Boulder, CO 80309, USA. tenio.popmintchev@jila.colorado.edu.
authors.forename: Tenio
authors.initials: T
authors.lastname: Popmintchev