As a critical optical component in laser systems, the performance of high-reflective coatings directly influences the output power of the entire laser system. Particularly in high-power laser applications, the laser-induced damage threshold (LIDT) of these coatings is a key limiting factor. The angle of laser incidence significantly affects the LIDT of coatings. Numerous studies have explored the influence of laser incident angle on temperature distribution, electric field (E-field) distribution, and LIDT of high-reflective coatings. However, most samples are designed for specific incident angles. To enhance performance, it is essential to design coating structures based on practical usage angles. Through systematic theoretical analysis and experimental research, understanding the relationship between high-reflective coatings designed for different angles and their LIDTs can provide valuable insights for designing these coatings and selecting optimal incident angles in laser system configurations.

High-reflective coatings (R_s≥99.5% at the center wavelength of 1064 nm) are designed and deposited using electron beam evaporation on substrates, both with and without pre-planted nodule seeds, for different incident angles. The finite element method (FEM) is employed to simulate the E-field distributions of the coatings under their respective design angles and working laser wavelengths, as well as the localized E-field distribution optimized by nodule defects of varying diameters. The nanosecond and picosecond LIDTs of the samples are measured in accordance with ISO 21254 standards. The transmittance spectrum of the coatings is measured using a spectrometer (Lambda 1050 UV/VIS/NIR, Perkin-Elmer), and the reflectance spectrum is calculated while neglecting absorption. The surface figure of the coatings is characterized using an optical interferometer (ZYGO Mark Ⅲ-GPI). The root-mean-square (RMS) roughness of the coatings is measured with an atomic force microscope (AFM, Veeco Dimension-3100).

FEM simulation results show that the E-field distribution within the high-reflective coatings closely correlates with the design incident angle and nodule seed diameter (Fig. 2). The peak E-field intensity increases with the design incident angle, and the localized E-field enhancement is more pronounced at nodule defects with larger seed diameters (Fig. 3). Experimental results indicate that the LIDT (1053 nm, 8.6 ps) of the high-reflective coating increases with the design incident angle, partly due to the decrease in peak E-field intensity (Fig. 6). The damage morphology induced by laser irradiation at near-LIDT fluence manifests as isolated pits. As the laser fluence intensifies, so too does the density of damage pits, which eventually results in substantial damaged spots (Fig. 8). For high-reflective coatings deposited on substrates pre-planted with nodule seeds (diameter: 1000 nm), the initial damage closely correlates with the localized E-field enhancement at the nodule defect (Fig. 9). Under laser irradiation with a pulse width of 10 ns, the typical damage morphology includes a nodule-related pit surrounded by plasma scalds. The damage initiation position corresponds to the enhanced E-field distribution of the outermost SiO₂ layer at the nodule dome. Under laser irradiation with a pulse width of 8.6 ps, the damage initiation position corresponds to the enhanced E-field distribution of the outermost SiO₂ layer at the nodule dome and the middle area of the nodule boundary opposite the laser incident direction (Fig. 11). Further characterization of cross-section damage morphology demonstrates that damage in the middle region of the nodule boundary initiates at lower laser fluence compared to damage at the nodule dome (Fig. 10).

High-reflective coatings are designed with different laser incident angles. The effects of the design incident angle and nodule defects on the E-field distribution, as well as the laser-induced damage properties of high-reflective coatings, are theoretically and experimentally compared. FEM simulation results show that the E-field distribution closely relates to the design incident angle and the diameter of nodule defect seeds. For a given target reflectivity, high-reflective coatings with larger design incident angles exhibit lower peak E-field intensities. Nodule defects cause localized E-field enhancement, with larger seed diameters leading to more marked enhancements. Experimental results demonstrate that the design incident angle noticeably affects the nanosecond and picosecond LIDTs of high-reflective coatings, and the laser damage morphology correlates closely with the E-field distribution within the coating. In coatings with pre-existing nodule seeds, initial damage induced by nanosecond lasers appears on the outermost SiO₂ layer of the nodule dome, while damage induced by picosecond lasers appears at the midpoint boundary of the nodule defect. This research serves as a valuable reference for designing high-power laser coatings and laser systems.