Criterion of "sharp focusing" for a laser breakdown by a giant YAG laser pulse

An investigation was made of the influence of the focusing conditions on the breakdown in K8, KV, TF8, GLS21P, and ZhS11 glasses and in lithium niobate and tantalate crystals subjected to 20-ns pulses at the wavelength of 1.06 μm. The size dependences of the breakdown threshold power and intensity were obtained for the radiation focused into a spot of diameters ranging from 6.75 to 42.7 μm. The breakdown in all these materials was found to be due to self-focusing of the incident radiation. The condition of "sharp focusing" for the laser breakdown in these materials was deduced.