Effect of laser intensity on temporal and spectral features of laser generated acoustic shock waves: ns versus ps laser pulses.

Evolution of acoustic shock wave (ASW) properties generated during nanosecond (ns) and picosecond (ps) laser-induced breakdown (LIB) of atmospheric air at different input intensities is presented. The intensity is varied by changing the focal geometry of ns and ps pulses. The ASW pressures are observed to follow the dynamic interplay between the plasma density and recombination of plasma species. The conversion of laser energy to acoustic energy has increased from loose to tight focusing conditions. The central frequencies have moved toward the lower side with increasing laser intensities for both ns-LIB (76-48 kHz) and ps-LIB (111.2-92.1 kHz). The angular distribution of acoustic emissions was observed to follow the laser-induced plasma spark in both ns- and ps-LIB.