| In this work, the laser-induced breakdown mechanism that leads to nanoparticle formation was investigated with optical experiments and theoretical simulations. In the experiments, pulsed-laser radiations at 1.06 μm was used to ablate 20 μm glass microspheres at three different fluences, 3, 6, and 12 J/cm2. A frequency-doubled laser beam illuminated the generated shockwave profile in a Schlieren configuration with delay times up to 30 ns. Shockwave images verified that breakdown in a 20 μm glass microsphere was initiated near the back surface due to internal focusing of the laser. The breakdown became the source for shockwave formation. Over the range of delays studied, shockwave velocity was approximately constant. The measured shockwave velocities were proportional to applied laser fluence and the velocity of the forward shockwave was faster than that of the backward (toward the laser source) shockwave. For longer times and propagation distances, however, we would expect the shockwave velocity to decrease. Therefore, measurements at longer optical delay times are needed to develop a complete shockwave propagation model for laser ablation of a glass microspheres.; A hydrodynamic numerical code was developed to further investigate the optical breakdown and shockwave initiation process. It included a two fluid model (electrons and ions) to account for energy exchange, energy radiation, and thermal conduction due to interactions between and among electrons and ions. A quotidian equation of state was implemented to overcome nonphysical data from the SESAME equation of data tables, especially in the low temperature and pressure region. Numerical simulations showed density, pressure, temperature, and velocity profiles. Simulation results with 20 μm glass microspheres confirmed the location of the initial breakdown and matched experimental shockwave velocities.; Nanoparticle size controllability was examined for permalloy and silver by changing the laser fluence, gas pressure and gas type. It was found that nanoparticle sizes were proportional to applied laser fluence. Further experimental and theoretical studies are suggested to better understand the laser ablation mechanism and the formation of nanoparticles. |
