Laser damage studies of 351-nm HR silica-zirconia sol-gel coatings

Zirconia and silica sol gels have been spin and dip coated on 50mm diameter fused silica substrates to produce 351nm HR multilayer stacks and very high laser damage thresholds. Multilayer coating design software has been used to estimate the refractive indices of individual layers. A spectrophotometer has been used to evaluate the absorption coefficient of the zirconia layers in the UV region. The coatings were typically found to craze after about 6 to 9 layer pairs had ben deposited. Laser damage thresholds up to 8J/cm2 have been measured using a frequency tripled Nd:YAG laser at 351nm and 0.7ns pulse duration using constant and ramped fluence sequences. The coatings and laser damage morphologies have been evaluated using optical, interferometric and electron microscope techniques.