Extended phase matching of high harmonic generation by plasma-induced defocusing

High-harmonic generation (HHG) of femtosecond lasers produces unique short-wavelength light pulses with femtosecond to attosecond duration. However, free electrons in the partially ionized gas medium are known to prevent phase matching and limit upconversion to low-energy harmonics only. We have demonstrated experimentally and theoretically an unconventional phase-matching scheme: extending the HHG phase matching cutoff by controlling the rapid self-defocusing effect of the driving laser. This method takes advantage of the additional intrinsic atomic dipole phase mismatch introduced by the rapid laser defocusing. This phase can be precisely controlled by adjusting the aperture of a simple iris, which truncates the input beam, to correctly compensate free-electron dispersion, resulting in tunable harmonic energy and phase matching cutoff extension. Based on this approach, we report experimental observation of extending the harmonic cutoff energy in Ar to ~65 eV with a 400-fold increase in conversion efficiency using a tightly focused truncated Gaussian beam in a short high-pressure gas cell. This new defocusing-assisted phase matching in a highly ionized gas medium can be applied to different targets, laser wavelengths, and pulse durations to extend harmonic upconversion to higher cutoff energy with higher efficiency.