Role of thermal self-focusing in optical breakdown of transparent dielectrics by nanosecond pulse fields

A theoretical analysis is made of the influence of self-focusing of light on the optical breakdown of transparent materials by focused beams. Special attention is paid to the influence of thermal self-focusing. It is shown that this effect is not usually the dominant mechanism of beam contraction in optical breakdown experiments. This is because, in contrast to the instantaneous Kerr mechanism, the diffraction in thermal self-focusing can compensate the nonlinear refraction; in some cases, thermal diffusion may also be important. The methods for experimental determination of the dominant self-focusing mechanism in optical breakdown are analyzed. It is pointed out that, in many cases, the necessary information can be obtained from measurements of the dependence of the breakdown threshold on the focal length of a lens.