What Is Optical Thin Film?

A class of optical dielectric materials composed of thin layered media that propagate light beams through the interface. The application of optical films began in the 1930s. In modern times, optical films have been widely used in the fields of optics and optoelectronic technology to manufacture various optical instruments.

Optical film

A class of thin layered media that propagates beams through interfaces

The main optical thin film devices include reflective films,

The characteristics of optical films are: smooth surface, and the interface between the film layers is geometrically divided; the refractive index of the film layer can change at the interface, but is continuous within the film layer; it can be a transparent medium or

Optical films are divided into reflective films by application,

The application of optical film is everywhere, from glass coating to mobile phone, computer, TV LCD display to LED lighting, etc. It is full of all aspects of our life and makes our life more colorful.

The definition of an optical film is: a thin and uniform dielectric film layer that is attached to the surface of an optical device during the propagation path of light, and the reflection, transmission (reflection) and

Veitch Tech's liquid crystal display optical film is an optical film formed by the principle of multiple refraction and focusing of light generated by microstructures. Its unique technology and process reduce light absorption and ensure light penetration and higher brightness. In addition to improving the brightness benefits, it can also achieve the effect of light diffusion and fogging through the refraction and scattering of light.

Brightening film

Brightening film (BEF) is an optical film on the surface of PET with excellent transparency. It uses acrylic resin to precisely shape a uniformly dispersed prism structure and a light diffusion layer on the back. The microstructure recovers and concentrates light to produce a brightening effect, high-brightness design, and a diffusion function. Due to the basic principle of the diffusion layer, the phenomenon of light coupling is eliminated, and the light display is more uniform and soft.

Diffuser

The diffusion sheet (DL series) is an acrylic resin on the surface of PET with excellent transparency, which is used to precisely coat a layer of randomly dispersed microstructured particles, and a layer of randomly dispersed microstructure is precisely coated on the opposite side of PET. The anti-static particles are used in liquid crystal displays to cause light to be refracted and diffracted multiple times through the diffusion layer, so as to equalize the light and make the light display more uniform and soft.

Reflective film

Reflective sheet is a film that is doped with HR polymer optical agent and plasticizer in PET resin in order to achieve light-shielding and high-reflective effect during the casting method. Because the intermediate layer of the film has a certain degree of light absorption, The reflection effect is reduced. Therefore, adding a layer of HR dielectric film on the surface to achieve better reflection effect and have anti-yellowing function.

Simple models of optical films can be used to study their general properties such as reflection, transmission, phase change, and polarization. If you want to study the special properties of optical films such as loss, damage, and stability, simple models are powerless. At this time, the crystal structure, internal structure and surface state of the film, the anisotropy and heterogeneity of the film, and the Composition, surface contamination, interfacial diffusion, and more. After considering these factors, it is necessary to consider not only its optical properties, but also its physical properties,

Optical film Chemical, mechanical, and surface properties, as well as penetration and influence between various properties. Therefore, the study of optical thin films has jumped out of the field of optics and has become a fringe discipline of physics, chemistry, solids and surface physics.

Although the optical phenomenon of thin films has been noticed as early as the 17th century, special research on optical films as a subject has begun after the 1930s, mainly because the development of vacuum technology has provided various optical film preparations. prerequisites. Today, optical films have been greatly developed, and the production of optical films has gradually moved towards serialization, programming, and specialization. However, there are still many problems in the research of optical films that need to be further resolved. The level has not been able to meet the requirements in many tasks and needs to be improved. In theory, not only the growth mechanism of the thin film needs to be clarified, but also the optical theory of the thin film, especially the optical theory applied to the very short wavelength band, needs to be further improved and improved. In terms of technology, people still lack effective means to achieve precise control of thin-film deposition parameters. In this way, the growth of thin-film

Optical film With a certain degree of randomness, the optical constant of the film, the thickness of the film, and the performance of the film also have a certain degree of instability and blindness, all of which limit the improvement of the quality of the optical film. As far as the optical film itself is concerned, in addition to the improvement of optical performance and the reduction of light loss such as absorption and scattering, its mechanical strength, chemical stability and physical properties need to be further improved. In the laser system, the anti-laser intensity of the optical film is low, which is one of the most important issues in the research of optical films. The following describes several commonly used optical film elements.

Optical film antireflection film

Also called antireflection coating, its main function is to reduce or eliminate the reflected light from optical surfaces such as lenses, prisms, and flat mirrors, thereby increasing the amount of light transmitted by these elements and reducing or eliminating stray light from the system.

The simplest anti-reflection coating is a single-layer film, which is a low-refractive-index film coated on the optical surface of an optical part. When the refractive index of the film is lower than that of the base material, the reflection coefficients r 1 and r 2 of the two interfaces have the same

Optical film Phase change. If the optical thickness of the film layer is a quarter of a certain wavelength, the optical path difference between two adjacent beams of light is exactly , that is, the vibration direction is opposite, and the superimposed result reduces the optical surface's reflected light at that wavelength. The refractive index of the film is appropriately selected so that r 1 and r 2 are equal, and the reflected light from the optical surface can be completely eliminated at this time.

Under normal circumstances, it is difficult to achieve the ideal anti-reflection effect with a single-layer antireflection coating. In order to achieve zero reflection at a single wavelength or achieve a good anti-reflection effect in a wide spectral region, double-layer, three-layer or even Multi-layer antireflection film. Figures a, b, and c plot the remaining reflection curves of the single-layer, double-layer, and triple-layer AR coatings on the surface of Kg glass, respectively.

Optical film

Anti-reflection film is the most widely used and largest output optical film. Therefore, it is still an important research topic in optical film technology. The focus of research is to find new materials, design new film systems, improve the deposition process, It uses the least number of layers and the simplest and most stable process to obtain the highest possible yield and achieve the best results. For the laser thin film, the anti-reflection film is the weak link of the laser damage. How to increase its damage strength is also one of the most concerned issues.

Optical thin film reflective film

Its function is to increase the reflectivity of the optical surface. Reflective films can generally be divided into two categories, one is a metal reflective film, and the other is a full dielectric reflective film. In addition, there is a metal dielectric reflective film that combines the two.

Optical film Generally, metals have a large extinction coefficient. When a light beam is incident on the metal surface by air, the amplitude of the light entering the metal rapidly decays, so that the light energy entering the metal decreases correspondingly, and the reflected light energy increases. The larger the extinction coefficient, the faster the light amplitude decays, the less the light energy entering the metal, and the higher the reflectivity. People always choose those metals with larger extinction coefficients and more stable optical properties as metal film materials. Metal film materials commonly used in the ultraviolet region are aluminum, aluminum and silver are commonly used in the visible region, and gold, silver and copper are commonly used in the infrared region. In addition, chromium and platinum are also commonly used as film materials for some special films. Because aluminum, silver, copper and other materials are easily oxidized in the air to reduce performance, they must be protected with a dielectric film. Common protective film materials include silicon oxide, magnesium fluoride, silicon dioxide, aluminum oxide, and the like. The advantage of 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 cannot be very high. In order to further improve the reflectivity of the metal reflective film, several dielectric layers with a certain thickness can be plated on the outside of the film to form a metal dielectric reflective film. It should be pointed out that the reflective film of a metal dielectric increases the reflectivity of a certain wavelength (or a certain wave region), but destroys the characteristics of neutral reflection of the metal film.

Optical film The all-dielectric reflective film is based on multi-beam interference. In contrast to an anti-reflection coating, a thin film with a higher refractive index than the base material can be used to increase the reflectivity of the optical surface. The simplest multilayer reflective film is formed by alternating evaporation of two materials with high and low refractive index, and the optical thickness of each layer is a quarter of a certain wavelength. Under this condition, the reflected light vectors on the interfaces participating in the superposition have the same vibration direction. The composite amplitude increases with the number of thin film layers. Figure 2 shows how the reflectivity of such a reflective film changes with the number of layers.

In principle, the reflectivity of the all-dielectric reflective film can be infinitely close to 1, but the scattering and absorption losses of the thin film limit the improvement of the thin film reflectance. So far, although the reflectivity of high-quality laser reflective films has exceeded 99.9%, some work still requires its reflectivity to continue to increase. The reflection film applied to the strong laser system emphasizes its resistance to laser light more. The work around improving the resistance to laser light of such films makes the research of such films more in-depth.

Optical thin film interference filter

It is a kind of optical film with the largest variety and complicated structure. Its main function is to split the spectral band. The most common interference filters are cut-off filters and band-pass filters. The cut-off filter can divide the spectrum under consideration into two parts, one part does not allow light to pass (referred to as the cut-off region), and the other requires light to pass fully (referred to as the band-pass region). According to the position of the passband in the spectral region, it can be divided into two types: long pass and short pass. Their simplest structures are as follows, where H and L represent thick high and low refractive index layers, and m is the number of cycles. The film system having the above structure is called a symmetric periodic film system. If the spectral region under consideration is wide or the ripple requirements of the passband transmittance are high, the film structure will be more complicated.

Bandpass filters allow only one section of the spectral band to pass, and all other sections are filtered out. According to their structure, they can be divided into Fabry-Perot filters, multi-cavity filters and induced AR Filter. Fabry-Perot filter

Optical film The structure is the same as the Fabry-Perot etalon (see Fabry-Perot interferometer), because the transmission spectral bands obtained by it are relatively narrow, so it is also called a narrow-band interference filter. The transmittance of this filter is very sensitive to the loss of the thin film, so it is difficult to prepare a filter with a high transmittance and a narrow half-width. Multi-cavity filters are also called rectangular filters, which can be used as narrow-band band-pass filters and broadband-pass filters. Film is equally difficult.

Induced AR filters are a type of filters that are matched on both sides of the metal film with an appropriate dielectric film system to increase the potential transmittance, reduce reflections, and increase the passband transmittance. Although its passband performance is not as good as that of the all-dielectric method-Perot filter, it has a wide cutoff characteristic, so it still has great application value. Especially in the ultraviolet region, the superiority of the dielectric material is more obvious when its absorption is relatively large. Figures a, b, and c show typical curves of Fabry-Perot filters, multi-cavity filters, and AR filters, respectively.

Optical thin film

A thin film that splits the beam into two parts according to certain requirements and a certain way. The spectroscopic film mainly includes several types, such as a wavelength spectroscopic film, a light intensity spectroscopic film, and a polarization spectroscopic film.

Wavelength beamsplitter film is also called two-color beamsplitter film. As the name suggests, it is a thin film that divides the light beam into two parts according to the wavelength region. This film can be a cut-off filter or a band-pass filter. The difference is that the wavelength splitting film must consider not only transmitted light but also reflected light, both of which require a certain shape of the spectral curve. The wavelength splitting film is usually used at a certain incident angle. In this case, due to the influence of polarization, the spectral curve will be distorted. In order to overcome this effect, the problem of depolarization of the film must be considered.

The light intensity splitting film is a thin film that divides the light beam into two parts according to a certain light intensity ratio.

Optical film The film sometimes considers only a certain wavelength and is called a monochromatic spectroscopic film; sometimes it is necessary to consider a spectral region called a broadband spectroscopic film; the broadband spectroscopic film for visible light is also called a neutral spectroscopic film. This film is also often applied under oblique incidence. Due to the influence of polarization, the polarization states of the two beams of light can be greatly different. In some work, this difference can be ignored, but in other work (such as some interferometers) ), Both beams are required to be depolarized, which requires the design and preparation of a depolarizing film.

The polarization beam splitting film is made by using the polarization effect of the thin film when the light is incident obliquely. Polarizing beam splitting film can be divided into two types: prism type and flat type. The prism-type polarizing film uses the polarization effect of the interface when the Brewster angle is incident (see the refraction and reflection of light on the interface). When the light beam is always incident at the interface between two materials at the Brewster angle, the reflected light of horizontal vibration is always zero regardless of the number of film layers, and the light of vertical component vibration increases with the number of film layers. With the increase, as long as the number of layers is sufficient, the transmitted light beam can be basically light that vibrates in the parallel direction, and the reflected light beam is basically light that vibrates in the vertical direction, so as to achieve the purpose of polarization beam splitting. This type of Brewster angle at the interface of the thin film material, so the thin film must be plated on the prism, at this time the incident medium is not air but glass. The flat-type polarizing film is mainly made by using the difference in the bandwidths of the reflection bands of the two polarization components of the dielectric reflecting film in oblique incidence. Generally, with the increase of the incident angle, the reflection bandwidth of the vertical component gradually increases, while the bandwidth of the parallel component gradually decreases. Selecting a highly reflective area with a vertical component and a highly transparent area with a parallel component as the working area can form a polarizing film that reflects the vertical component through the parallel component. The incident angle of this polarizing film is generally selected near the Brewster angle of the substrate . A prism-type polarizing film has a wide working wavelength range and can be made with a high degree of polarization, but it is troublesome to prepare, difficult to be made large, and resistant to laser light. The flat-type polarizer works in a relatively narrow wavelength region, but it can be made very large and has a high resistance to laser light, so it is often used in high-power laser systems.

Fig. 4 and Fig. 5 respectively show the reflection spectrum curves of the neutral light splitting film and the flat polarizing splitting film.

Optical film