Correlation of plasma ion densities and phase matching with the intensities of strong single high-order harmonics

We studied the correlation between the intensity of the strong quasi-monochromatic harmonics (13th harmonic of indium and 17th harmonic of tin) of an 800 nm laser pulse and the characteristics of the plasma in which the harmonics were generated. By varying the intensity of the prepulse that produces the plasma from a solid target, we found an optimum plasma condition under which the intensity of the strong harmonics reached maximum. Fluid simulations suggest that this behaviour is correlated to the maximum density of singly charged ions, which are thought to be responsible for the generation of the strong single harmonics via specific plasma resonances. This result is consistent with the model recently proposed by Milosevic (2007 J. Phys. B: At. Mol. Opt. Phys. 40 3367). We also examined the alternative mechanism of phase matching between the harmonics and the pump laser to explain the role of plasma resonances in the enhancement effect. An optimum plasma condition is found as in the experiments although the relation between the strong single harmonic intensity and the singly charged ion density is not straightforward. The ac Stark broadening of the plasma resonances appears to be a critical parameter for phase matching to occur.