The relative roles of avalanche multiplication and multiphoton absorption in laser-induced damage of dielectrics

The optical electric field strengths associated with pulsed laser exposures needed to produce conduction electron densities of 10¹⁸/cm³in several direct-gap alkali halides are calculated using three different models: a simplified avalanche model, the Keldysh formulation of multiphoton ionization, and a combination of the two. Numerical calculations are performed for crystalline NaCl, KCl, KBr, NaF, LiF, and CaF2at wavelengths of 1.064, 0.694, 0.532, and 0.355 μm, for nanosecond and picosecond pulse durations. The results are compared with available experimental data resulting in the following observations: the damage field strengths predicted by the avalanche model scatter around the experimentally measured values, but they always agree within a factor of approximately four. The electric field strengths required for breakdown solely from the simultaneous absorption of four or more photons are significantly larger than the experimental values or the predictions of the avalanche model. However, in NaCl, KCl, and KBr the electric fields necessary for damage due to four-photon absorption are slightly smaller than those needed for catastrophic avalanche multiplication, and are in significantly closer agreement with the experimentally measured damage thresholds. When the avalanche and multiphoton models are combined in a direct manner the resulting thresholds are close to the smaller of the two previously calculated thresholds, and are in reasonable agreement with the experimental data with respect to their dependence on laser frequency and pulse duration.