Neutral depletion and beam defocusing in harmonic generation from strongly ionized media
Аннотация
We present a detailed experimental study of the effects of strong ionization in the process of medium-order harmonic generation. The intensity-dependent density of free electrons significantly modifies the conversion efficiency due to the combined effect of depletion of the neutral atoms and defocusing of the pump beam. We find that the harmonic yield is maximized for particular values of the pump-laser intensity and that, for the range of experimental parameters we used, such optimum intensities do not depend on the harmonic order or on the focusing geometry, but only on the ionization potential of the gas. Finally we show how the defocusing due to ionization is effective in guiding the pump beam along channels of optimum intensity, thus enhancing the overall conversion efficiency for particular focusing conditions. A simple model is presented that, though relying on very crude approximations of the processes at play, is nevertheless able to reproduce the experimental results in a surprisingly good way.