A first-order model for computation of laser-induced breakdown thresholds in condensed media

An analytic, first-order model has been developed to calculate irradiance thresholds for laser-induced breakdown (LIB) in condensed media, including fluids and ocular media. The model is derived from the simple rate equation formalism of Shen for cascade breakdown in solids and from the theory of multiphoton ionization in condensed media developed by Keldysh. Analytic expressions have been obtained for the irradiance thresholds corresponding to multiphoton breakdown, to cascade breakdown, and to initiation of cascade breakdown by multiphoton ionization of seed electrons (multiphoton initiation threshold). The model has been incorporated into a computer code and code results compared to experimentally measured irradiance thresholds for breakdown of ocular media, saline, and water by nanosecond, picosecond, and femtosecond laser pulses in the visible and near-infrared. Theoretical values match experiment to within a factor of 2 or better, over a range of pulsewidths spanning five orders of magnitude, a reasonably good match for a first order model.