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.

Effect of pulse duration on the acoustic frequency emissions during the laser-induced breakdown of atmospheric air.

Acoustic shock waves (ASWs) in the frequency range of 30-120 kHz generated during laser-induced breakdown (LIB) of ambient air using 7 ns and 30 ps pulse durations are studied. The specific frequency range and peak amplitudes are observed to be different for nanosecond (ns) and picosecond (ps) LIB. The ASW frequencies for ps-LIB lie between 90 and 120 kHz with one dominant peak, whereas for ns-LIB, two dominant peaks with frequencies in the 30-70 kHz and 80-120 kHz range are observed. These frequencies are observed to be laser pulse intensity dependent. With increasing energy of ns laser pulses, acoustic frequencies move toward the audible frequency range. The variation in the acoustic parameters, such as peak-to-peak pressures, signal energy, frequency and acoustic pulse widths as a function of laser energy, for two different pulse durations are presented in detail and compared. The acoustic emissions are observed to be higher for ns-LIB than ps-LIB, indicating higher conversion efficiency of optical energy into mechanical energy.

Spatio-temporal dynamics behind the shock front from compacted metal nanopowders.

Laser ablated shock waves from compacted metal nanoenergetic powders of Aluminum (Al), Nickel coated Aluminum (Ni-Al) was characterized using shadowgraphy technique and compared with that from Boron Potassium Nitrate (BKN), Ammonium Perchlorate (AP) and Potassium Bromide (KBr) powders. Ablation is created by focused second harmonic (532 nm, 7 ns) of Nd:YAG laser. Time resolved shadowgraphs of propagating shock front and contact front revealed dynamics and the precise time of energy release of materials under extreme ablative pressures. Among the different compacted materials studied, Al nanopowders have maximum shock velocity and pressure behind the shock front compared to others.

Spatio-temporal dynamics behind the shock front from compacted metal nanopowders.

Laser ablated shock waves from compacted metal nanoenergetic powders of Aluminum (Al), Nickel coated Aluminum (Ni-Al) was characterized using shadowgraphy technique and compared with that from Boron Potassium Nitrate (BKN), Ammonium Perchlorate (AP) and Potassium Bromide (KBr) powders. Ablation is created by focused second harmonic (532 nm, 7 ns) of Nd:YAG laser. Time resolved shadowgraphs of propagating shock front and contact front revealed dynamics and the precise time of energy release of materials under extreme ablative pressures. Among the different compacted materials studied, Al nanopowders have maximum shock velocity and pressure behind the shock front compared to others.