Neutral depletion and beam defocusing in harmonic generation from stronglyionized media - art. no. 023411

We present a detailed experimental study of the effects of strong ionizatio n in the process of medium-order harmonic generation. The intensity-depende nt density of free electrons significantly modifies the conversion efficien cy due to the combined effect of depletion of the neutral atoms and defocus ing of the pump beam. We find that the harmonic yield is maximized for part icular values of the pump-laser intensity and that, for the range of experi mental parameters we used, such optimum intensities do not depend on the ha rmonic order or on the focusing geometry, but only on the ionization potent ial of the gas. Finally we show how the defocusing due to ionization is eff ective in guiding the pump beam along channels of optimum intensity, thus e nhancing the overall conversion efficiency for particular focusing conditio ns. A simple model is presented that, though relying on very crude approxim ations of the processes at play, is nevertheless able to reproduce the expe rimental results in a surprisingly good way.