Dependence of high-order harmonic intensity on the length of preformed plasma plumes

An experimental study has been carried out on high-order harmonic generation from the interaction of $45\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ Ti:sapphire laser pulses with low-excited silver plasma plumes. The dependence of high-order harmonic intensity $({I}_{H})$ on the plasma plume length $({L}_{\mathrm{med}})$ shows slower than quadratic scaling ${I}_{H}\ensuremath{\sim}{L}_{\mathrm{med}}^{p}$ with ``$p$''$\ensuremath{\approx}0.9$, 0.8, and 0.7 for 21st, 33rd, and 41st harmonics, respectively. The harmonic intensity is observed to decrease with increasing harmonic order, and this decrease is faster for longer plasma plumes. The results are explained from physical considerations of phase mismatch between the driving laser pulse and the harmonic radiation during propagation, and reabsorption of the harmonic radiation in the plasma plume.

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