The main optical coating devices include reflective film, anti-reflection film, polarizing film, interference filter and beam splitter, etc. They have been widely used in the national economy and national defense construction, and have received increasing attention from scientific and technical workers. For example, the use of anti-reflection coatings can reduce the luminous flux loss of complex optical lenses ten times; the use of mirrors with high reflective film ratios can double the output power of lasers; the use of optical coatings can improve the efficiency and efficiency of silicon cells. stability.
The simplest optical coating model is a uniform dielectric film with a smooth and isotropic surface. In this case, the optical interference theory can be used to study the optical properties of optical coatings. When a beam of a monochromatic light plane wave is incident on the optical coating, multiple reflections and refractions occur on its two surfaces. The directions of the reflected light and the refracted light are given by the law of reflection and the law of refraction, and the amplitude of the reflected light and the refracted light The size is determined by the Fresnel formula.
The optical coating can be divided into: reflective film, anti-reflection film/anti-reflection film, filter, polarizer/polarizing film, compensation film/phase difference plate, an alignment film, diffusion film/sheet, according to its use classification, characteristics and application. Brightness enhancement film/prism film/condenser film, shading film/black and white glue, etc. Related derivative types include optical grade protective film, window film and so on.
The characteristics of optical coating are: the surface is smooth, and the interface between the layers is geometrically divided; the refractive index of the layer can jump at the interface, but it is continuous in the layer; it can be a transparent medium or an absorption Medium; it can be uniform in the normal direction or non-uniform in the normal direction. Practical coatings are much more complicated than ideal coatings. This is because: during preparation, the optical and physical properties of the coating deviate from the bulk material, and the surface and interface are rough, which leads to the diffuse reflection of the light beam; the interpenetration between the film layers forms a diffuse interface; due to the growth of the film layer , Structure, stress and other reasons, formed various anisotropies of the coating; the coating has a complicated time effect.
Reflective films can generally be divided into two categories, one is the metal reflective film, and the other is all-dielectric reflective film. In addition, there is also a metal-dielectric reflective film that combines the two, the function is to increase the reflectivity of the optical surface.
Generally, metals have a large extinction coefficient. When the light beam is incident on the metal surface from the air, the amplitude of the light entering the metal is rapidly attenuated, so that the light energy entering the metal is correspondingly reduced, and the reflected light energy is increased. The greater the extinction coefficient, the faster the light amplitude decays, the less light energy entering the metal, and the higher the reflectivity. People always choose metals with larger extinction coefficients and stable optical properties as metal film materials. The commonly used metal plating materials in the ultraviolet region are aluminum, aluminum and silver are commonly used in the visible light region, and gold, silver and copper are commonly used in the infrared region. In addition, chromium and platinum are often used as special coating materials. Since aluminum, silver, copper and other materials are easily oxidized in the air and reduce their performance, they must be protected by a dielectric film. Commonly used protective film materials include silicon monoxide, magnesium fluoride, silicon dioxide, and aluminum oxide.
The advantage of the metal reflective film is that the preparation process is simple, and the working wavelength range is wide; the disadvantage is that the light loss is large, and the reflectivity can not be very high. In order to further improve the reflectivity of the metal reflective film, several dielectric layers of a certain thickness can be plated on the outer side of the film to form a metal-dielectric reflective film. It should be pointed out that the metal-dielectric film increases the reflectivity of a certain wavelength (or a certain wave region), but destroys the neutral reflection characteristics of the metal film.