Optical detection of laser plasma interaction during laser ablation

Laser plasma interaction during pulsed laser ablation is investigated by ultrafast phototube detection. There are two peaks in an optical signal with the first peak attributed to laser scattering and the second one to plasma generation. As laser fluence increases, the second peak rises earlier to overlap with the first one. The signal is fitted by different distribution functions for the laser scattering and the plasma generation. Peak amplitude and its arrival time, full width at half maximum (FWHM), starting time and termination time of the distributions are studied for different laser fluences and detection angles. Laser pulse is mainly scattered from the plasma during the laser ablation. Peak amplitude of the laser scattering increases but its FWHM decreases with laser fluence. Angular distribution of the peak amplitude can be fitted with cos(n) theta (n=4) while detection angle has no obvious influence on the FWHM. In addition, FWHM and peak amplitude of the plasma increase with laser fluence. However, its starting time and peak arrival time reduce with laser fluence. Time interval between plasma starting and scattered laser pulse termination is proposed as a quantitative parameter to characterize laser plasma interaction. Threshold fluence for the interaction can be estimated to be 3.5 J/cm(2) for KrF excimer laser ablation of silicon. For laser fluence above 12.6 J/cm(2), the plasma and scattered laser pulse distributions tend to saturate.