Deformation of intense laser beams tightly focused inside NaCl: A comparison of the multiphoton-polaron and avalanche models of optical breakdown

Computer simulations of the interaction of intense picosecond laser pulses (
\lambda = 532
nm) with NaCl in the focal volume of a 2.54 cm focal length aberration-free lens indicate that severe beam deformation may take place at photon fluxes corresponding to the single-shot damage threshold measured under these experimental conditions. This deformation is the result of a change in the dielectric function which, in turn, is caused by free carriers generated via multiphoton-assisted avalanche ionization and/or multiphoton absorption. Strong self-defocusing prevents local photon fluxes from reaching values required to raise the lattice temperature sufficiently for damage to occur. We conclude that either both the semiempirical avalanche ionization as well as the multiphoton-polaron absorption mechanisms of laser breakdown are invalid in their present form or that the experimentally determined damage thresholds do not pertain to the intrinsic properties of NaCl.