Picosecond pulse train and nanosecond pulse were compared for laser ablation and laser induced breakdown spectroscopy (LIBS) measurements. A detailed study revealed that the picosecond pulse train ablation improved the quality of laser craters (symmetric crater walls and the absence of large redeposited droplets), which was explained by a smaller heat affected zone and suppression of melt splash. Greater plasma dimensions and brighter plasma emission were observed by gated imaging for picosecond pulse train compared to nanosecond pulse ablation. Increased intensity of atomic and ionic lines in gated and time integrated spectra provided better signal-to-noise ratio for picosecond pulse train sampling. Higher temperature and electron density were detected during first microsecond for the plasma induced by the picosecond pulse train. Improved shot-to-shot reproducibility for atomic/ionic line intensity in the case of picosecond pulse train LIBS was explained by more effective atomization of target material in plasma and better quality of laser craters. Improved precision and limits of detections were determined for picosecond pulse train LIBS due to better reproducibility of laser sampling and increased signal-to-noise ratio.
A new method of non-linear laser spectroscopy based on four-wave scattering of pump laser radiation into the probe laser cavity is suggested and realized. As a result of intracavity amplification there is an opportunity to investigate weak narrow resonances of different types including the resonances of photo-elastic scattering in optical materials and laser crystals. A theory describing this phenomenon is suggested and experiments with narrowband (1 MHz linewidth) tunable continuous-wave diode laser were carried out. It is shown that the four-wave scattering of pump laser radiation into the cavity of the probe laser can be used for measurement of dispersion of photo-elastic constants in solid and liquid dielectrics.
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