Systematic investigation on light intensification by typical subsurface cracks on optical glass surfaces

By using the finite-difference time-domain method, a systematic investigation has been performed to analyze the light field redistribution due to the modulation from typical subsurface cracks on optical glass surfaces. The involved cracks that can occur during surface processing are mainly of three basic types, including the cone, the radial, and the lateral. It is found out that the modulated light intensification strongly depends on the crack type and especially on if the crack is on the front or rear surface. If the cone crack is on the rear surface or lateral crack is on the front surface, a strong field enhancement as large as 2 orders of magnitude can occur. As for the other four cases, i.e., cone crack on the front surface, lateral crack on the rear surface, and radial crack on both the front and rear surfaces, they do not result in a field enhancement of 2 orders of magnitude, but only several times higher than the incident light field. The field enhancement mechanisms for various crack types are compared and discussed in detail. If only from the perspective of laser damage initiation resulting from a modulated high electric field, the most harmful subsurface cracks may be initially the lateral on the front surface and the cone on the rear surface.