Phase-matching optimization of large-scale nonlinear frequency upconversion in neutral and ionized gases

We suggest and discuss two techniques that could radically improve the efficiency of large-scale nonlinear frequency upconversion: quasi-phase matching of high-order harmonic generation in density-modulated media and high-order difference-frequency mixing in plasma. We demonstrate theoretically that both techniques permit optimal phase matching with tight focusing of pump beams, which is detrimental to current approaches to frequency upconversion, with a potentially drastic increase in conversion efficiency. Optimal phase-matching conditions that we obtained in the perturbation limit are shown to be likely to hold for strong pumping fields as well.