We present our results on spatio-temporal evolution of laser plasma produced shockwaves (SWs) and hot core plasma (HCP) created by focused second harmonic (532 nm, 7 ns) of Nd-YAG laser in quiescent atmospheric air at f/#10 focusing geometry. Time resolved shadowgraphs imaged with the help of an ICCD camera with 1.5 ns temporal resolution revealed the presence of two co-existing sources simultaneously generating SWs. Each of the two sources independently led to a spherical SW following Sedov-Taylor theory along the laser propagation direction with a maximum velocity of 7.4 km/s and pressure of 57 MPa. While the interaction of SWs from the two sources led to a planar SW in the direction normal to the laser propagation direction. The SW detaches from the HCP and starts expanding into the ambient air at around 3 µs indicating the onset of asymmetric expansion of the HCP along the z-axis. The asymmetric expansion is observed till 10 µs beyond which the SW leaves the field of view followed by a deformation of the irradiated region in the XY-plane due to the penetration of surrounding colder air in to the HCP. The deformation in the XY-plane lasts till 600 µs. The dynamics of rapidly expanding HCP is observed to be analogous to that of cavitation bubble dynamics in fluids.

A two-dimensional axis symmetric hydrodynamic model was developed to investigate nanosecond laser induced plasma and shock wave dynamics in ambient air over the input laser energies of 50–150 mJ and time scales from 25 ns to 8 μs. The formation of localized hot spots during laser energy deposition, asymmetric spatio-temporal evolution, rolling, and splitting of the plasma observed in the simulations were in good agreement with the experimental results. The formed plasma was observed to have two regions: the hot plasma core and the plasma outer region. The asymmetric expansion was due to the variation in the thermodynamic variables along the laser propagation and radial directions. The rolling of the plasma was observed to take place in the core region where very high temperatures exist. Similarly, the splitting of the plasma was observed to take place in the core region between the localized hot spots that causes the hydrodynamic instabilities. The rolling and splitting times were observed to vary with the input laser energy deposited. The plasma expansion was observed to be asymmetric for all the simulated time scales considered, whereas the shock wave evolution was observed to transfer from asymmetric to symmetric expansion. Finally, the simulated temporal evolution of the electron number density, temperature of the hot core plasma, and the temperature evolution across the shock front after the detachment from the plasma were presented over the time scales 25 ns–8 μs for different input laser pulse energies.

The dynamics of tightly focused ultrashort (40 fs) pulses manifested in terms of supercontinuum emission (SCE) and cavitation-induced bubbles (CIB) resulting from propagation in water over a wide range of input powers (6 mW-1.8 W) are presented. The effect of linear polarization (LP) and circular polarization (CP) on SCE in different external focal geometries (f /6, f /7.5 and f /10) is investigated and the results are discussed. SCE with higher efficiency and a considerable spectral blue shift is observed under tight focusing conditions (f /6) compared to loose focusing conditions (f /10). At higher input powers, CIB along the axis of propagation are observed to be assisting deeper propagation of these short pulses and enhanced SCE.

Ablative plasma and a shock wave (SW) in ambient air were experimentally produced using Nd:YAG laser pulses of ∼7 ns width and a wavelength of 532 nm. The numerical simulations of the experiments were performed using a two-dimensional axis-symmetric radiation-hydrodynamics code. The numerical approach to simulate the experimental observations was not straightforward due to the complex behavior of the laser-air interaction and the associated processes, such as plasma formation and SW evolution, that occur concurrently. Hence, the modeling was attempted based on the combination of two laser absorption coefficients and two equations-of-state (EOSs). One form of absorption coefficient was taken from Zel'dovich and Raizer [Physics of Shock Waves and High-Temperature Hydrodynamic Phenomena (Dover Publications/Academic Press Inc., New York, 2012)], which is the sum of photoionization and inverse bremsstrahlung (IB) due to electron-ion collisions, and the other was taken from DeMichelis [IEEE J. Quantum Electron. 5(4), 188 (1969)] that considers the IB due to electron-ion and electron-neutral collisions. Similarly, the two EOSs, namely the ideal gas EOS and the chemical equilibrium application [S. Gordon and B. J. McBride, NASA Ref. Publ. 1311, 1 (1994)] EOS, are considered. The simulated results obtained using four models were compared with each other and with the experimental observations. These models enabled understanding the transient behavior of the laser-induced air plasma and the SW evolution. The results showed that the absorption coefficient and the EOS play a key role in modeling the dynamics of air plasma and SW. We present the results of this study and the models which validate the experimental results the best in terms of the asymmetric plasma expansion, formation of hot spots, plasma splitting and rolling, SW external dynamics such as the transition from a tear-drop to a spherical shape, and shock front velocity.

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.