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An integrated analysis of laser induced breakdown (LIB) and the associated micro-cavitation in a water-like bio-tissue is developed in this study. With this integrated analysis using minimum experimentally fitted parameters, threshold energies of LIB in bio-tissue under different numerical apertures of the focusing lenses can be determined, which provides similar values as the corresponding experimental measurements. The integrated analysis can also determine the electron temperature and number density when LIB is triggered, which also fall in the same order as the corresponding experimental measurements. In addition, the rapid size increment of plasma plume through laser supported detonation wave during a LIB can be observed with the integrated analysis. The developed integrated analysis becomes unstable when strong pressure wave presents due to strong heating and the following rapid expansion of the bio-tissue under higher input laser energy (compared with the threshold energy of LIB). Therefore, for simulating high energy LIB and the following micro-cavitation in a bio-tissue, a more advanced numerical scheme that can better capture the propagation of pressure wave will be required in the future.

With high spatial but low temporal coherent nanosecond (ns) green light, ns pulsed illumination phase-contrast microscopy is successfully constructed. The pulsed illumination phase-contrast microscopy can provide ns time resolution to resolve transient dynamics of transparent targets with high contrast ratio that cannot be achieved with traditional bright-field or phase-contrast microscopies. Transient dynamics of laser-induced microcavitation is successfully recorded with this imaging technique to demonstrate its capabilities. In addition to tracing the evolutions of vapor bubbles and shockwaves during a laser-induced microcavitation, this imaging technique can sense the refractive index change of the transparent target during a pressure field variation.

Laser-induced microcavitation refers to the rapid formation and expansion of a vapor bubble inside the bio-tissue when it is exposed to intense, pulsed laser energy. With the associated microscale dissection occurring within the tissue, laserinduced microcavitation is a common approach for high precision bio-surgeries. For example, laser-induced microcavitation is used for laser in-situ keratomileusis (LASIK) to precisely reshape the midstromal corneal tissue through excimer laser beam. Multiple efforts over the last several years have observed unique characteristics of microcavitions in biotissues. For example, it was found that the threshold energy for microcavitation can be significantly reduced when the size of the biostructure is increased. Also, it was found that the dynamics of microcavitation are significantly affected by the elastic modules of the bio-tissue. However, these efforts have not focused on the early events during microcavitation development. In this study, a direct numerical simulation of the microcavitation process based on equation of state of the biotissue was established. With the direct numerical simulation, we were able to reproduce the dynamics of microcavitation in water-rich bio tissues. Additionally, an experimental setup in deionized water and 10% PAA gel was made to verify the results of the simulation for early micro-cavitation formation for 10% Polyacrylamide (PAA) gel in deionized water.

A numerical analysis capable of describing the early stage of a thermal microcavitation process in a water-rich biotissue without avalanche breakdown was developed. The analysis successfully reproduced the laser-induced heating, vapor bubble formation, bubble expansion, and shockwave propagation inside a water-rich biotissue during a thermal microcavitation process. Based on the analysis, it was determined that the evolution of the temperature, pressure, and laser-induced shockwave is dependent on the incident laser energy and laser pulse width. On the other hand, the early stage dynamics of the microcavitation process showed little dependence on the elastic modulus of the biotissue for the laser and tissue conditions studied.