Role of light intensification by cracks in optical breakdown on surfaces

The intensity distribution of an initially plane light wave incident on pla nar and conical surface cracks is calculated numerically by using a wave pr opagation computer code. The results show that light intensity enhancements caused by interference of internal reflections at the crack and the surfac e are very sensitive to the light polarization, the beam angle of incidence , and the crack geometry (e.g., crack width and orientation with the surfac e). The light intensity enhancement factor (LIEF) can locally reach 2 order s of magnitude for conical cracks of ideal shape. The electric field direct ion relative to the crack surfaces determines the light intensity profile a round the crack. For normal-incidence illumination on the output surface, t otal internal reflection at the crack and the surface can occur and leads t o higher LIEFs. For identical geometry and illumination conditions, a crack located on the entrance surface of an optic generates electric field enhan cements that are weaker than those on the exit surface. As cracks on polish ed surfaces are randomly oriented, the probability for large intensity enha ncements to occur is high. The model is able to predict quantitatively the magnitude of surface laser-induced damage threshold drop and damage propaga tion enhancement in dielectric materials that are due to cracks. (C) 2001 O ptical Society of America.