An experimental study of high-order harmonic generation in In, InSb, InP, and InGaP plasmas using femtosecond laser radiation with variable chirp is presented. Intensity enhancement of the 13th and 21st harmonics compared to the neighboring harmonics by a factor of 200 and 10, respectively, is observed. It is shown that the harmonic spectrum from indium-containing plasma plumes can be considerably modified by controlling the chirp of the driving laser pulse.

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It is shown that the influence of atomic autoionizing states on the phase matching of high harmonic generation results in efficient selection of the single harmonic generated in the plateau region. The change of relative concentration of the atomic medium components and variation of the fundamental laser frequency can be used for tuning of the selected harmonic frequency. The possibility of achievement of contrast for the selected single harmonic of more than 10(4) at intensity conversion efficiency of approximately 10(-3) has been shown.

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