TEM Investigation of Effects of a Barrier Layer on Damage to 1.064µ AR Coatings

Silica/titania 4 layer AR coatings on BK-7 glass, with and without a half-wave silica barrier layer, have been damaged at 1.064µ by 150 ps laser pulses. The damage sites have been examined by TEM replica techniques at a magnification of 70,000. The size of the damage sites appears to be “quantized.” Most sites consist of a 1µ nucleus which leads to a 3µ crater in a fully developed site. The mechanical properties of the films appear to play a role in the morphology of the damage sites. The titania layers seem to fracture while the silica layers seem to melt or tear. A series of micrographs of near threshold damage sites shows the development of the sites from a rupture which appears to occur at or near the substrate surface. Evidence of heating in the substrate is also present, although it is difficult to pinpoint the exact starting point of the damage mechanism.