Phase matching, quasi-phase matching, and pulse compression in a single waveguide for enhanced high-harmonic generation

We demonstrate, for the first time to our knowledge, that the efficient region of high-harmonic generation can be shifted from lower to higher photon energies by combining phase matching, quasi-phase matching, and pulse compression in a single gas-filled waveguide. An intrawaveguide pulse compression process that works through a combination of ionization-induced refraction and guiding shortens the laser pulse as it propagates through an Ar-filled waveguide. This leads to enhanced harmonic emission at high photon energies near 95 eV while it reduces emission at low photon energies near 45 eV. The waveguide geometry also mitigates ionization-induced refraction, allowing Ar gas with high effective nonlinear susceptibility to be used.