Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas

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.The experimental work was done at JILA, supported by Army Research Office grant WN11NF-13-1-0259, an NSF PFI AIR award, and U.S. Department of Energy (DOE) grant DE-SC0008803 (M.M.M., T.P., and H.C.K.). Theory was supported by a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013) under REA grant agreement 328334 (C.H.-G.); Junta de Castilla y León (SA116U13, UIC016) and MINECO (FIS2013-44174-P) (C.H.-G. and L.P.); NSF grants PHY-1125844 and PHY-1068706 and AFOSR MURI “Mathematical Modeling and Experimental Validation of Ultrafast Light-Matter Coupling associated with Filamentation in Transparent Media” grant FA9550-10-1-0561 (A.J.-B., R.J.L., X.G., A.L.G., M.M.M., and H.C.K.); Ministry of Science and Technology, Taiwan, grant 102-2112-M-007-025-MY3 (M.-C.C.); U.S. Department of Energy, Division of Chemical Sciences, Atomic, Molecular and Optical Sciences Program (A.B.); and DOE Office of Fusion Energy, HED Laboratory Plasmas program, grant AT5015033 (S.B.L., M.F., and J.A.G.). Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security LLC for DOE, National Nuclear Security Administration, under contract DE-AC52-07NA27344, LLNL-JRNL-676693. T.P., D.P., M.M.M., and H.C.K. have filed a patent on “Generation of VUV, EUV, X-ray Light Using VUV-UV-VIS Lasers,” U.S. patent application 61873794 (2013)/US 20150063385 (2015)

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

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