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

High harmonic up-conversion 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 up-conversion to low energy harmonics only. Here we report experimental observation of extending the harmonic cutoff energy in Ar to ~ 65eV with a 400-fold increase in conversion efficiency using a tightly focused truncated Gaussian beam in a short high pressure gas cell. Analysis of three dimensional propagation simulation reveals that an unexpected new type of dynamic phase matching mechanism is at play where plasma dispersion from high ionization can be balanced by the large change of intrinsic high-harmonic phase initiated by plasma-induced defocusing itself. The method is robust and can be applied to different targets, laser wavelengths and pulse durations to optimize the generation of high energy harmonics conveniently by adjusting the iris opening before the beam enters the gas cell. This new phase matching mechanism in a highly ionized gas medium offers new opportunities for further investigations to extend harmonic up-conversion to higher cutoff energy with higher efficiency.