|
The major physical reason for laser induced damage of real dielectric materials, their surfaces, and thin films is laser heating of the absorbing inclusions /1/. The role of the inclusions in the process of damage initiation is well experimentally established for both bulk damage /2-5/, and surface damage of transparent dielectrics /6-8/ and thin films /9/. The theoretical models of laser damage associated with the absorbing inclusions have been considered in /6,10-15/. The model of thermal explosion most consistently considered in /14-15/ for the case of bulk damage is evidently most adequate for the description of laser damage physics.
Despite the general nature of thermal explosion of absorbing
inclusions located in the dielectric bulk, surface layer, or thin
film, their characteristics (threshold temperature Te and threshold
intensity Ie) may differ. To investigate these differences one has
to take into account numerous and, as a rule, purely controllable
factors. For instance, it is well known that physico-chemical properties of the dielectric surface layer strongly differ from its
properties in the bulk, the degree of difference depending on the
surface treatment techniques. For this reason, a detailed investigation of the differences in the inclusion thermal explosion in the bulk and dielectric surface layer should be performed parallel with control of physico-chemical properties of this layer.
However, in order to clear up ultimate resistance of the dielectric
material surface, one has to study thermal explosion of the inclusion located in a perfect dielectric surface layer, whose physico-chemical properties coincide with those of the bulk. In the present paper we determine a relationship of the thermal explosion characteristics for the inclusions in the bulk and on a perfect surface, analyze thermal explosion kinetics, study the dielectric surface laser damage statistics with due regerd to the thermal explosion kinetics, and determine the dependences of the surface damage threshold Isd, upon laser pulse width end spot size. The results of the work are compared with experimental data.
|
