A graded-optical-bandgap coating structure for high-power picosecond laser mirrors

The laser-induced damage threshold (LIDT) of mirror coatings under picosecond laser irradiation is closely related to the high-refractive-index (n) materials employed. Materials with relatively higher n values tend to exhibit narrower optical bandgaps and are more susceptible to damage under laser irradiation, particularly in regions of high electric-field intensity. To mitigate this issue, we propose a graded-refractive-index design strategy. Specifically, materials with relatively lower n values are used for high-n layers located in regions of intense electric-fields, while materials with gradually increasing n values are employed for high-n layers along the direction in which the electric-field intensity decays. Based on this strategy, we designed and prepared an M-Graded mirror using Al2O3 and Al2O3/HfO2 nano-laminates as the high-n layers. Compared with a conventional M-Constant mirror, which uses a fixed-n Al2O3–HfO2 mixture as the high-n material, the LIDT of the M-Graded mirror is increased by a factor of 1.49. Moreover, the bandwidth with reflectivity exceeding 99.5% is reduced by only approximately 6.9%. We believe this design strategy offers substantial advantages for various types of multilayer coatings used in high-power picosecond laser applications.