Threshold dependence of laser-induced optical breakdown on pulse duration

A model for optical breakdown is presented, that takes the effects of avalanche ionization, electron–ion recombination, and electron diffusion into account. By assuming a rectangular temporal shape of the laser pulse, an analytical solution of the threshold parameters can be given. Especially, a square root dependence of the energy density on the pulse duration is found for pulse duration values between roughly 4 ps and 8 μs. The theory explains the results of several previous experiments investigating threshold parameters for a variety of target materials.