PHASE-MATCHING OPTIMIZATION OF LARGE-SCALE NONLINEAR FREQUENCY UP-CONVERSION IN NEUTRAL AND IONIZED-GASES

We suggest and discuss two techniques that could radically improve the efficiency of large-scale nonlinear frequency upconversion: quasi-pha se matching of high-order harmonic generation in density-modulated med ia and high-order difference-frequency mixing in plasma. We demonstrat e theoretically that both techniques permit optimal phase matching wit h tight focusing of pump beams, which is detrimental to current approa ches to frequency upconversion, with a potentially drastic increase in conversion efficiency. Optimal phase-matching conditions that we obta ined in the perturbation limit are shown to be likely to hold for stro ng pumping fields as well. (C) 1996 Optical Society of America