Variation of harmonic spectra in laser-produced plasmas at variable phase of femtosecond laser pulses of different bandwidth
Abstract
The intensity enhancement of harmonics and change in harmonic profiles have been observed in a few plasma plumes (La, In, Mn) under variable phase modulation of Ti:sapphire laser pulses. It is seen that variation of relative harmonic intensities in the plateau region could be achieved by chirp variation of narrowband (~10 nm bandwidth) radiation. The effect of the self-phase modulation (SPM) of broadband (~20 nm bandwidth) laser radiation on the harmonic emission from nanoparticles, C60, and Ag plasma while passing through a glass slab is also studied. The observation of broadband harmonic emission and redshift in harmonic wavelengths for an unchirped laser pulse are explained in terms of SPM. The observation of blueshift of harmonic radiation with both positively and negatively chirped pulses was attributed to the different effects of SPM on the chirped pulses.