High-order harmonic generation in laser-produced ions using a near-infrared laser

We describe experimental studies of high-order harmonic generation in rare-gas-like ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ ions. The ions were created in a laser-produced plasma and the harmonics were generated with a short-pulse, high-power, titanium-sapphire laser operating at 794 nm. The highest harmonic order observed was the 27th (29.4 nm), while much higher harmonic orders were observed in a jet of neutral Ne gas under similar conditions. Pronounced ring structures were observed in the harmonic far-field distribution and these structures varied with focusing condition. The cutoff in the harmonic spectra at high orders may be due to ionization-induced defocusing of the fundamental laser beam. Calculations illustrating how this effect reduces the peak intensity obtained in the plasma compared with the intensity obtained in the Ne gas are presented.