Comparison of 355-nm nanosecond and 1064-nm picosecond laser-induced damage in high-reflective coatings

Abstract

The damage properties of multilayer coatings tested with 1064-nm 30-ps pulses are similar to those tested with 355 nm, nanosecond pulses. A kind of HfO2 / SiO2 high-reflective (HR) coating is prepared by electron beam evaporation. Laser-induced damage of HfO2 / SiO2 HR coatings is tested by 355-nm 7-ns pulses and 1064-nm 30-ps pulses, respectively. Damage morphologies and cross-sectional profiles are characterized using a scanning electron microscope and focused ion beam, respectively. The laser-induced damage thresholds and morphologies in the two tests are compared. The developing processes and damage mechanisms are discussed. Many similarities are found in the two tests: the typical damage morphologies in both tests appeared as micrometer-sized pits when irradiated by low-fluence pulses, while it turned out to be layer delamination when irradiated by high-fluence pulses. Damage onset is nearby the peak of the E-field in the two tests. Damage pits in both tests may be related to thermal stress caused by nanometer-sized isolated absorbers. There are also some differences in the damage properties between two tests: damage pits in 1064-nm 30-ps tests have a much higher density than that in 355-nm 7-ns tests. The detail features and the developing processes of the pits are different.