Impact of substrate surface scratches on the laser damage resistance of multilayer coatings

Abstract

Substrate scratches can limit the laser resistance of multilayer mirror coatings on high-peak-power laser systems. To date, the mechanism by which substrate surface defects affect the performance of coating layers under high power laser irradiation is not well defined. In this study, we combine experimental approaches with theoretical simulations to delineate the correlation between laser damage resistance of coating layers and the physical properties of the substrate surface defects including scratches. A focused ion beam technique is used to reveal the morphological evolution of coating layers on surface scratches. Preliminary results show that coating layers initially follow the trench morphology on the substrate surface, and as the thickness increases, gradually overcoat voids and planarize the surface. Simulations of the electrical-field distribution of the defective layers using the finite-difference timedomain (FDTD) method show that field intensification exists mostly near the top surface region of the coating near convex focusing structures. The light intensification could be responsible for the reduced damage threshold. Damage testing under 1064 nm, 3 ns laser irradiation over coating layers on substrates with designed scratches show that damage probability and threshold of the multilayer depend on substrate scratch density and width. Our preliminary results show that damage occurs on the region of the coating where substrate scratches reside and etching of the substrate before coating does not seem to improve the laser damage resistance.

Coupling effect of subsurface defect and coating layer on the laser induced damage threshold of dielectric coating

“…The research on the subsurface defect of optical component began in the early 20th century. To date, the major defect on substrate surface include scratch [1] [2], impurities [3] and so on. Numerous studies focused on understanding how the subsurface defect affect the laser induced damage performance of coating.…”

Laser-resistance sensitivity to substrate pit size of multilayer coatings

“…The damage mechanisms of these dielectric multilayers have been studied extensively to maximize the laser-induced damaged threshold (LIDT)3. Structural defects, such as scratches4, impurities56789, and pits10 on the substrate, are considered to be among the most important factors limiting the coating function and lifetime. Deformation in the film structure resulting from high or low points on the substrate surface (particles1112/artificial nodule seeds131415 or scratches4/pits610) yield enhancements in the electric-field (E-field) intensity and therefore reduce the LIDT.…”

“…Structural defects, such as scratches4, impurities56789, and pits10 on the substrate, are considered to be among the most important factors limiting the coating function and lifetime. Deformation in the film structure resulting from high or low points on the substrate surface (particles1112/artificial nodule seeds131415 or scratches4/pits610) yield enhancements in the electric-field (E-field) intensity and therefore reduce the LIDT. Because the multilayer dielectric coating growth exhibits a self-shadowing nature by rotating the substrate during electron-beam (EB) evaporation, the particle defects become nodules that are eventually laid on the surface14; however, the pits/scratches are usually buried under the multilayers and cannot be easily observed10.…”

“…Because the multilayer dielectric coating growth exhibits a self-shadowing nature by rotating the substrate during electron-beam (EB) evaporation, the particle defects become nodules that are eventually laid on the surface 14 ; however, the pits/scratches are usually buried under the multilayers and cannot be easily observed 10 . However, the pit defects have a relatively low density and random geometrical dimensions, making quantitative research difficult 4 10 . Considering the increasing need for large optics with high-power laser-damage resistance, it is very time-consuming and challenging to obtain representative damage morphologies of multilayer coatings on a pitted substrate and to make meaningful comparisons with simulation results.…”

Searching for optimal mitigation geometries for laser-resistant multilayer high-reflector coatings

“…The cross-sectional view of conical pit with rough edge was achieved by a dual beam focused ion beam milling technique [18]. Fig.…”