Formation of periodic microstructures on multilayer dielectric gratings prior to total ablation

The damage morphology produced by high-power, short-pulse lasers on multilayer dielectric (MLD) gratings has been closely examined. An unusual ripple formation arises under specific laser fluence conditions and produces a bright diffractive effect. A single irradiation does not produce this morphology, proving that it is a cumulative effect requiring multiple laser shots on a test site. The period of this microstructure is found to be between 2.0 and 2.4 mum. The ripple orientation varies across the test site. Varying several experimental conditions such as pulse length, beam polarization and angle of incidence still produces this periodic microstructure, though not always efficiently. This morphology is not seen on MLD stacks or other homogeneous samples.