Phase-matched third-harmonic generation in a gas-filled hollow waveguide

There has been great interest in the high-order harmonic generation (HHG) in gases as sources of novel tools for high photon energy pump-probe experiment (≈100 eV) with a great temporal resolution (<ps). In order to improve HHG efficiency, reduction of phase mismatch between the pump and harmonic waves, which lead to elongation of an interaction length, is essential. However, previous studies unfortunately lacked efforts to clarify a phase matching mechanism because the HHG experiments have been achieved with focused Gaussian beams in pulsed gas jets, which complicated the phenomena in the interaction region. We propose HHG in a hollow waveguide and demonstrate phase-matched third-harmonic generation of ultrashort laser pulses for the first time