Microscopic simulation of short pulse laser damage of melanin particles

Microscopic mechanisms of short pulse laser damage to melanin granules, the strongest absorbing chromophores of visible and near - IR light in the retina and skin, are studied using the molecular dynamics simulations. The pulse width dependence of the fracture/cavitation and vaporization processes within the small particles, their coupling to the surrounding medium and the resulting tissue injury are discussed based on the simulation results. The effect of laser irradiation on an isolated submicron particle at different laser fluences and pulse durations is first analyzed. The mechanical disruption of the particle due to the laser induced pressure is found to define the character of damage for short pulse widths (tens of picoseconds) at laser fluences that are significantly lower than those required for boiling. Thermal relaxation and explosive disintegration of the overheated particle at higher laser fluencies are the processes that dominate at longer laser pulses (hundreds of picoseconds). Damage of an absorbing particle embedded into a transparent medium with different mechanical characteristics is then simulated. Coupling of the acoustic and thermal pulses generated within absorbing particles to the surrounding medium is studied and the possible cumulative effects from an ensemble of absorbing particles are discussed. The simulation results provide the basis for future work in which the microscopic and continuum descriptions are combined for multiscale modeling of laser tissue interaction.