Enhanced high-order harmonic generation from Xe, Kr, and Ar in a capillary discharge

We report the use of a preionized medium created by a capillary discharge to extend the cutoff photon energy in high-order harmonic generation. The observed enhancements result from a combination of reduced ionization energy loss and reduced ionization-induced defocusing of the driving laser. We observe harmonic emission from Xe up to a photon energy of 160 eV, and we extend this technique to other noble gases, observing photons with energies up to 170 eV from Kr and 275 eV from Ar. The discharge plasma also provides a means to spectrally tune the harmonics by tailoring the initial level of ionization of the medium. Our results are interpreted using a hydrodynamic-atomic physics model of the discharge plasma. This work demonstrates that capillary discharges are a versatile and general method for generating harmonics, in particular from ions. Finally, this approach should be scalable to efficiently generate coherent light at much shorter wavelengths, in combination with phase-matching techniques.