Classification of optical coatings and their characteristics
Optical manufacturing is developing rapidly in mainland China. China's optical manufacturing has begun to take an important position in the international economic arena, and the country has become one of the top producers of optical glass and optical components.
Optical thin film is an indispensable part of various advanced optoelectronic technologies, which not only improves the system performance, but also is necessary to meet the design goals, optical coatings are broadly divided into two types of metal coatings and dielectric films (transparent film). Either one can be used as a reflector for optical experiments, but the mechanism of reflection is completely different, and there are many differences in the methods of use or characteristics.
Metallic film is a reflector with high reflectivity when aluminum (Al) or gold (Au) is vaporized on a polished glass substrate. In addition, metals such as silver (Ag), platinum (Pt), or chromium (Cr) can sometimes be used for mirrors. Metal films are capable of reflecting over a very wide wavelength spectrum and are characterized by a small change in reflectivity with the angle of incidence. As the unreflected light is absorbed by the metal film, the light will not be able to reach the glass substrate through the film when the thickness of the metal film becomes slightly thicker.
Aluminized film
Aluminum films have high reflectivity in the ultraviolet to infrared spectral regions, but are very susceptible to oxidation, especially in the ultraviolet region, and have unstable properties. Moreover, it is easily damaged and the surface cannot be wiped even if it gets dirty. Therefore, by attaching a protective film to the aluminum film, the metal film can be protected and prevented from oxidation or damage. The protective film has the effect of maintaining reflectivity in certain wavelength spectral regions, but in other wavelength spectral regions, the reflectivity is sometimes reduced. In contrast, there are also special protective films that increase reflectivity only in specific wavelength spectral regions.
Gold plated film
It has yellow wavelength characteristics (blue absorption) in the visible spectrum, but has a high reflectivity in a very wide range of the infrared spectrum. Since only the gold film does not adhere well to the glass surface, it is easy to peel off, so it is usually first vaporized with a chromium base film. The gold film is soft and easily damaged. In the specific wavelength spectrum, we can vaporize the protective film of gold film. For use in the entire infrared spectrum, gold films without a protective coating are often used. Do not wipe the gold film with paper or cloth. Once it is damaged, it cannot be recovered.
Chromium Plated Film
Chromium film or its alloy (chromium-nickel-iron alloy) can be used as an optical coating for some reflectors. The reflectivity of chromium film is lower than aluminum film and gold film, due to more absorption and not used for reflectors, but in the wide wavelength spectrum because of the reflectivity and absorption rate of change is small, can be used for reflective neutral filters or beamsplitters dielectric film dielectric material colorless and transparent, no reflection or absorption as large as the metal. If proper material and film thickness are selected, interference effect will be produced at the interface of glass substrate, film and air, and specific transmittance and reflectance wavelength characteristics can be obtained. The wavelength characteristics of reflectivity.
Single layer reflective film
When light is directed into the glass substrate, a reflection of about 4% is generated, resulting in a loss of transmittance. However, the reflectivity of the glass substrate can be changed by vaporizing a dielectric film on the glass substrate with a lower refractive index than that of the glass.
If the thickness of the dielectric film is adjusted so that its optical range (refractive index n×film thickness d) is λ/4, the reflections at the interface between the glass substrate and the dielectric film, the dielectric film and the air can be offset and the reflectivity can be reduced to a minimum.
However, the refractive index is limited by the film material, so the reflectivity cannot be completely zero. Moreover, not all glass substrates can be antireflective due to the limitation of refractive index of the glass substrate.
Multi-layer antireflection film
Since the choice of single-layer film material is very small and the glass substrate has some residual reflection. Therefore, by overlapping several layers of films, the best anti-reflection effect can be achieved even with a small number of film materials.
In addition, the composition of the film can be changed to produce narrow-band antireflection films (NMAR) that reduce reflectivity at specific wavelengths, or broad-band antireflection films (MLAR) that reduce reflectivity in the broad wavelength spectrum.
Multi-layer reflective film
By alternately and repeatedly vaporizing a dielectric film with a high refractive index and a dielectric film with a low refractive index on a glass substrate, a very high reflective film can be obtained.
The interface between the high refractive index and the low refractive index will produce very little reflection.
Since the thickness of each layer of the dielectric film is adjusted to λ/4 of the optical range (refractive index n×film thickness d), the light reflected on each layer has the same phase, and the reflections will be synthesized and strengthened by each other. On the contrary, the light rays advancing in the direction of transmission through multiple reflections will cancel each other and become zero.
If the number of layers of the dielectric film is large enough, the incident light will gradually weaken and become almost impermeable.
All the attenuated light will be converted to reflected light. Since there is no absorption in the electrolyte film, the incident light will not be lost and will become 100% reflected light.
