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Chinese Coating
Optical elements vapor-deposited with various thin films were used in optical experiments. In many cases, thin films are evaporated onto polished glass surfaces. Many necessary optical properties such as reflectivity, wavelength characteristics, polarization characteristics, etc. in optical experiments are achieved by coating. Therefore, in order to give full play to the performance of components such as mirrors, the light is directly injected into the side coated with the film. Coatings are roughly classified into two types: metal coatings and transparent films (dielectric films).
Any kind of mirror can be used for optical experiments, but the reflection mechanism is completely different, and there are also many differences in the method of use or characteristics.
Metal Coating
When a metal such as aluminum (Al) or gold (Au) is vapor-deposited on a polished glass substrate, it becomes a mirror with high reflectivity. In addition, metals such as silver (Ag), platinum (Pt), or chromium (Cr) are sometimes used for mirrors. The metal film can reflect in a very wide wavelength spectrum and has the characteristic that the reflectivity changes little with the incident angle. Since the unreflected light is absorbed by the metal film, when the thickness of the metal film becomes slightly thicker, the light cannot pass through the film to reach the glass substrate.
Aluminum Coating
Aluminum coating has high reflectivity in the ultraviolet to infrared spectral region, but it is very easy to oxidize, especially in the ultraviolet spectral region, which has unstable characteristics. Moreover, it is easily damaged, and the surface cannot be wiped even if it is soiled. Therefore, adding a protective film to the Aluminum coating can protect the metal coating and prevent oxidation or damage. The protective film has the effect of maintaining the reflectance in a specific wavelength spectrum, but in other wavelength spectrums, the reflectance may decrease. Contrary to this, there are also special protective films that increase reflectivity only in specific wavelength spectrum regions.
Chrome Coating
Chromium coating or their alloys (Inconel) can be used as coatings for partial mirrors. The reflectivity of chromium coating is lower than that of Aluminum coating and gold coating. It is not used for mirrors because of more absorption. However, in the wide wavelength spectrum region, due to the small changes in reflectivity and absorption, it can be used for reflective neutral filter or beamsplitter.
Gold Coating
It has a yellow wavelength characteristic in the visible spectral region (blue has absorption), but has a high reflectivity over a very broad range of the infrared spectral region. Since only the gold coating cannot adhere well to the glass surface and is easy to peel off, the bottom coating of chrome is generally evaporated first. The gold coating is soft and easily damaged. When used in a specific wavelength spectrum region, the protective film of gold coating can be evaporated. Unprotected gold coatings are often used when used across the infrared spectrum. Please never wipe the gold coating with paper or cloth. Once damaged, it cannot be recovered.
Dielectric coating
Dielectric materials are colorless and transparent and do not reflect or absorb as much as metals do. If an appropriate material and film thickness are selected, an interference effect occurs at the interface between the glass substrate and the thin film and air, and wavelength characteristics of specific transmittance and reflectance can be obtained.
When the light enters the glass substrate, it will produce about 4% reflection and cause the loss of transmittance. However, the reflectance of the glass substrate can be changed by vapor-depositing a dielectric film having a lower refractive index than glass on the glass substrate. When the thickness of the dielectric coating is adjusted so that the optical path (refractive index n×film thickness d) is λ/4, the reflection at the interface between the glass substrate and the dielectric coating, and between the dielectric coating and air can be canceled, and the reflectance can be minimized. However, since the refractive index is limited by the coating material, the reflectance cannot be completely zero. Moreover, due to the limitation of the refractive index of the glass substrate, not all glass substrates can obtain the anti-reflection effect.
Due to the small selection of single-layer film materials, and the glass substrate will have some residual reflections. Therefore, the best anti-reflection effect can be obtained even with a small amount of coating material by superimposing several layers of thin coating deposition. In addition, the composition of the coatings can be changed to produce narrow-band anti-reflection coatings (NMAR) that reduce reflectivity at specific wavelengths, or broad-band anti-reflection coatings (MLAR) that reduce reflectivity over a broad wavelength spectrum.
When a dielectric coating with a high refractive index and a dielectric coating with a low refractive index are alternately and repeatedly deposited on a glass substrate, a reflective coating with very high reflectivity can be obtained. The interface of high and low refractive index produces very little reflection. Since the thickness of the dielectric coating of each layer is adjusted to the optical path of λ/4 (refractive index n×film thickness d), the phase of the light reflected on each layer is the same, and the reflections will be synthesized and strengthened. On the contrary, the light rays traveling in the transmission direction after multiple reflections cancel each other and become zero. If the number of layers of the dielectric coating is sufficient, the incident light will gradually weaken and become almost impermeable. The attenuated light will all be turned into reflected light. Since there is no absorption by the electrolyte membrane, the incident light will not be lost and become 100% reflected light.
Multilayer dielectric coatings can produce many wavelength characteristics and are used in a variety of optical instruments. On the other hand, in cutting-edge research, some very special multilayer dielectric coatings have thin-coating structures with more than 100 layers in order to obtain better optical properties.
If it is in the visible spectrum region, dozens of layers of dielectric coating can achieve the reflection effect. However, if the wavelength spectrum includes from ultraviolet to infrared, it is necessary to combine more than three multilayer coatings in the ultraviolet, visible and infrared spectral regions. As a result, the number of coating layers becomes extremely large, requiring correspondingly more sophisticated manufacturing techniques.
When a high-energy pulsed laser is injected into the multilayer dielectric coating, the energy at the interface of the film becomes extremely large, and the ordinary coating is destroyed by the laser energy. Therefore, it is necessary to reconsider the thin coating structure and thin coating material, and develop special multilayer coatings that are less susceptible to laser damage.
Low dispersion coating for femtosecond laser
When a femtosecond laser is injected into a multi-layer dielectric coating with a broad spectral range, the optical path through the film varies with wavelength, resulting in dispersion. The design of the low-dispersion coating for femtosecond laser requires that the optical path in the coating does not change with the wavelength, and the dispersion is reduced to a minimum. Furthermore, a thin coating construction that can withstand high energy is required.
Composite Coating
By adding metal coatings to multilayer dielectric coatings, optical elements with unprecedented properties can be fabricated. Because the multilayer dielectric coating and metal coating are mixed together, it is called a composite film. This coating is used in broadband unpolarized beamsplitters and bandpass filters, among others. Unfortunately, there is a small amount of light loss due to metal absorption.
A multilayer coating can have any wavelength characteristics of reflectance and transmittance, but on the other hand, it is also subject to many restrictions.
When the incident angle of light is changed, the wavelength characteristics of the transmittance and reflectance of the multilayer coating are changed. The relative thickness of each layer of the multilayer coating, when the oblique light is incident, the optical path of the light passing through the coating will be longer. Therefore, when the incident angle is 0 degrees (vertical), the wavelength characteristics of transmittance and reflectance are on the longest wavelength side, and when the incident angle increases, the wavelength characteristics shift to the shorter wavelength side. This phenomenon is known as the blue shift of multilayer coatings.
Since the thickness of the dielectric coating and the refractive index of the thin coating material change with temperature, the wavelength characteristics of the transmittance and reflectance of the multilayer coating also change. Since the temperature dependence varies with the manufacturing method of the coating and the coating material, it needs to be considered at the design stage of the coating. Please notify the usage environment and temperature fluctuation range in advance.
The optical properties of the multilayer dielectric coating may change over time depending on the usage environment. Although there are different manufacturing methods and film compositions, when exposed to high temperature and high humidity for a long time, the coating will expand and the wavelength characteristics will also change. Special attention should be paid to products whose transmittance and reflectance vary greatly depending on wavelength, such as dichroic mirrors and bandpass filters. When optical components are used in laboratories or installations, they should be kept at room temperature and low humidity. When not in use, please store in a dry storage box such as an electronic drying box.
When the optical element is used at an angle of incidence other than 0 degrees (normal incidence), polarization characteristics occur. The polarization characteristics include two characteristics: the change characteristics of transmittance and reflectance of P-polarized light and S-polarized light, and the change characteristics of the phase difference between P-polarized light and S-polarized light. Phase difference characteristics are difficult to control and are not guaranteed for the coated products in this catalog. For example, when linearly polarized light in the direction of 45 degrees enters the coated product, the polarization state of the outgoing light is no longer linearly polarized at 45 degrees, but elliptically polarized. However, when the P-polarized light and the S-polarized light are not separated, the transmittance and reflectance characteristics are not particularly affected when the average value of the light intensity of the P-polarized light and the S-polarized light of the outgoing light is considered. When light is incident on an uncoated glass substrate at an incident angle of 45 degrees, the reflectance of P-polarized light and S-polarized light are different. The same is true for each thin film interface of the coating. There is a difference in the reflectivity of P-polarized light and S-polarized light. For multilayer films, due to the large difference in transmittance and reflectance of P-polarized light and S-polarized light, the wavelength characteristics will also be affected. . Therefore, the wavelength characteristic graph of transmittance and reflectance in the catalog shows the characteristics of P-polarized light and S-polarized light. Even if the P-polarized light is not shown in the figure, the coated optical element with the characteristic curve of S-polarized light also has polarization characteristics. In this case, the characteristic curve shows the average value of P-polarized light and S-polarized light. Since most light sources other than lasers are unpolarized, a graph of the average value of P-polarized and S-polarized light can be used. However, when using a laser, because it is linearly polarized, whether to take the P polarization value or the S polarization value, or the value between the two needs to be determined according to the polarization direction. In particular, such as a beam splitting mirror, the transmission region and the reflection region are converted in a narrow wavelength spectrum. The conversion wavelength of S-polarized light and P-polarized light is different from transmission to reflection.
