For high-precision optical thin film devices, the scattering of optical surfaces has attracted widespread attention. When the beam enters the optical surface, the particles on the optical surface scatter the incident light and the scattered light deviates from the specular reflection direction, resulting in the loss of the effective optical signal of the system. Furthermore, when the scattered light reaches the detector surface through the optical system, it increases the stray light of the optical system, so that the influence of noise on the signal cannot be ignored and reduces the detection sensitivity of the optical system. In a high-energy laser system, even very weak light scattering influences the thermal absorption level and laser damage threshold of the film. In the reflective astronomical telescope system, the pollutants on the surface of optical elements reduce the optical performance of the devices and make the optical devices completely unusable.

Presently, commonly used methods for reducing the scattering of particulate pollutants on the surface of optical elements mainly require keeping the optical surface clean and reducing pollutants on the optical surface. However, improper cleaning may fail to remove pollutants from the optical surface, damage the optical surface and increase additional light scattering. Furthermore, the material cost of pollution control for the environment where the optical elements are located is also huge. In the optical thin film evaporation process, the film particles are evaporated to the substrate surface together with the film molecules, and dot protrusions are formed on the substrate surface. This results in increasing scattering on the surface of the optical element. Here, the scattering of surface particles cannot be eliminated by cleaning and environmental maintenance. From the thin film vector scattering theory, this paper reduces the scattering amount of particulate pollutants on the optical surface by optimizing the film design, which does not require keeping the optical surface clean and can effectively reduce the frequency of optical surface cleaning to maintain the optical performance of the optical surface.

This study is based on the vector scattering theory. First, we theoretically calculate the electric field intensity at the center of the spherical particle on the surface of SiO₂ film with different thicknesses on Al film surface. Second, we obtain the SiO₂ film thickness corresponding to the minimum value of electric field intensity at the spherical particle center and set it as the optimized thickness of the SiO₂ film protective layer. Then, the bidirectional reflection distribution function and total scattering loss of spherical particulate pollutants on the Al film surface without a protective layer and with an optimized SiO₂ film protective layer are analyzed. Next, we compare the scattering amount of spherical particle pollutants on the surface of aluminum film without a designed protective layer and with an optimized SiO₂ protective layer. Finally, we conduct experiments to verify the effectiveness of this method.

To reduce the optical scattering of particulate pollutants on the surface of metal with highly reflective films, first, we analyze the electric field intensity at the center of spherical particulate pollutants on the surface of SiO₂ protective film with different thicknesses on the surface of the metal aluminum film (Fig. 3). Next, we take the physical thickness (125 nm) of the SiO₂ film corresponding to the minimum value of electric field intensity as the optimized thickness of SiO₂ film. Then, we theoretically calculate the bidirectional reflection distribution function (B_R) of the aluminum film coated with the optimized SiO₂ protective film when the beam is normally incident (Fig. 4). The results show that in the scattering angle of -90°-90°, the pollutant scattering on the surface of the optimized coating is effectively reduced, and B_R×cos θ_s in the reflected light direction without and with the optimized coating is 0.00855 and 0.00048, respectively, where θ_s is the scattering angle. Furthermore, the total scattering losses of pollutants on the surfaces of the original and optimized membrane systems are calculated as 0.018609 and 6.09264×10^-4 (Fig. 5), respectively, and the total scattering of particles on the surface of the optimized membrane system is reduced by 96.73%. Finally, four coatings are prepared experimentally, and the scatterings of pollutants on the surfaces of the four coatings are measured under the same pollution conditions. The results also confirm the effectiveness of the optimized SiO₂ film protection layer in reducing the scattering of particulate pollutants (Fig. 7).

The conclusions drawn from this paper are as follows. 1) The electric field intensity of particulate pollutants can be changed by changing the thickness of the SiO₂ film protective layer on the surface of the metal Al film. Compared with that without coating on the Al film surface, the square of the normalized electric field intensity at the center of the particle is effectively reduced using the optimized thickness of the SiO₂ film protective layer. 2) Taking the square of the normalized electric field intensity at the center of the spheroidal particle pollutant as the evaluation index and the physical thickness of SiO₂ film corresponding to the minimum value of this index as the optimized thickness of SiO₂ film, the light scattering of the pollutant with radius of 100 nm can be reduced significantly. 3) When plating a metal Al high reflector, the thickness of the SiO₂ protective film is a physical quantity that cannot be ignored and is related to the scattering amount of the metal reflector surface. The appropriate thickness of the SiO₂ protective film is greatly significant to reduce the surface scattering of metal mirrors, whereas improper thickness increases the surface scattering of metal mirrors.