Ultrafast laser-induced plasma diagnostics with time-spatial-resolved shadow and interferometric techniques

摘要

The experimental studies of laser-induced plasma accompanying the laser ablation of materials have been conducted with the developed shadow-interferometric technique. High intensity single picosecond pulses of YAP:Nd laser (λ=1078 nm) were applied to ablate tested samples and time-delayed probing pulses of second harmonic (λ=539 nm) illuminating the interaction area were used to make snap-shots of the expanded plasma plume. Both shadow and interferometric images of hot plasma were captured simultaneously with a CCD camera providing ≈ 1.5 micron spatial and ≈ 10 ps temporal resolution of the investigated processes. By varying the intensity of ablating pulses (in the range of 5·10~(13)/5·10~(14) W/cm~2) and the time-delay of probing pulses (in the ps range) it was possible to study a highly inhomogeneous small-scaled plasma density and refractive index distribution within the plume. The longer (nano-subnanosecond) time-delays allowed study of laser-initiated shock wave expansion in the surrounding atmosphere. A special attention was paid to the plasma formation arising at a through-hole laser drilling process. The strong influence of laser breakdown of the ambient gas (air) and laser-ignited explosion of clusters of ablated material on the efficiency of the drilling process was observed. In particular, it was shown that the cluster explosion can efficiently block the laser radiation resulting in decreasing the ablation rate. A computer modeling of optical visualization of small-scale plasma objects has been conducted. The analysis of the experimental and numerical results has revealed a number of characteristic features of plasma images that should be taken into account at the qualitative and quantitative evaluations of the plasma parameters.