High-order harmonic generation from the ablation of crystals

We analyze the generation of high-order harmonics during propagation of ultrafast pulses through the plasma plumes produced by laser ablation of the surfaces of fluorine- (NaF, CaF2, LiF, and MgF2) and oxygen- (KTiOPO4, BaB2O4, KH2PO4, and LiB3O5) containing crystals. The similarity of plasma and harmonic emission from the ablation of fluorine-containing crystals points out the same source (F II ions) of harmonics which was also supported by the coincidence of the harmonic cutoffs defined from experiment (55th-59th orders) and theory. The comparison of these results with ablation of Ca, Mg, and Li bulk targets showed that, in the latter case, the harmonic spectra became weaker with lower cutoffs, which also supports our assumption of the decisive role of fluorine ions in the higher-order harmonic generation. The ablation of oxygen-containing crystals resulted in formation of the plasmas allowing the lower-order and weaker harmonic generation compared with the case of F-containing plasmas. The difference in the cutoffs in that case pointed out different conditions of harmonic generation from the ablation of O-containing crystals, though the O II ions could be considered as the most probable sources of harmonics. (C) 2014 Optical Society of America