Optical Coating Technology: Introduction, Development and Future Trends 

Optical Coating Technology: Introduction, Development and Future Trends

Optical coating technology is a core technology that integrates multiple disciplines such as materials science, vacuum physics, and optical engineering. By depositing one or more layers of films on the surface of optical components, it precisely regulates the reflection, transmission, absorption, polarization, etc. of light, thereby enhancing the performance of optical systems and expanding their application scenarios. From everyday items like glasses and mobile phone cameras, to high-end laser equipment, space probes, and quantum communication devices, optical coating technology plays an irreplaceable role and is the "core foundation" of modern optoelectronic industries.

Optical coating refers to the process of depositing a layer (or multiple layers) of metal, medium or composite films on the surface of optical components through physical or chemical methods. The core purpose of this process is to modify the optical properties of the material surface to meet the usage requirements of different scenarios. In simple terms, optical coating is like "putting a special coat" on optical components. This "coat" is thin (with a thickness usually ranging from nanometers to micrometers), but it can achieve three core functions: first, it reduces the loss of light reflection and improves the transmittance of optical components (such as anti-reflective coatings on eyeglass lenses); second, it enhances the reflection ability of light and prepares high-reflection mirrors (such as lens plates in laser resonators); third, it realizes special functions such as splitting, filtering, and polarization of light (such as filter coatings on camera lenses and polarization coatings on AR glasses).

The core principle of optical coating is based on the interference effect of light. When light is incident on the surface of the film, it will undergo multiple reflections and transmissions on the upper and lower surfaces of the film layer, forming multi-beam interference. By precisely controlling the refractive index, thickness, and number of layers of the film layer, the superposition or cancellation of reflected light and transmitted light can be achieved, thereby achieving the expected optical effect. For example, anti-reflection films achieve this by designing single or multiple layers of medium films of specific thickness, allowing the reflected light to cancel each other out, allowing more light to pass through the component; high-reflection films, on the other hand, achieve this by the superposition of multiple layers of films, making the reflected light mutually enhance, achieving an extremely high reflectivity.

According to the basic principles of electromagnetism, the reflectivity and transmissivity of light can be calculated using formulas. When an air layer (with a refractive index of 1.0), a coating (such as a medium with a refractive index of 1.5) and glass (with a refractive index of 1.8) form an overlapping structure, the transmissivity can increase from around 85% without the coating to over 91%, fully demonstrating the core value of optical coating.

According to their functions, optical coatings can be classified into four main categories: The first is anti-reflection coatings (also known as reflective reduction coatings), which are used to reduce surface reflection and increase transmittance, and are widely applied in eyeglasses, camera lenses, optical windows, etc.; The second is high-reflection coatings, which are used to enhance light reflection and are applied in laser reflectors, solar reflectors, etc.; The third is filter coatings, which are used to filter specific wavelengths of light, such as infrared filter coatings and ultraviolet filter coatings, and are applied in security monitoring, medical imaging, etc.; The fourth is special function coatings, such as polarizing coatings, conductive coatings, self-cleaning coatings, etc., which are suitable for emerging scenarios like AR/VR and automotive optics.

According to the preparation process classification, the mainstream technologies can be divided into two major categories: physical vapor deposition (PVD) and chemical vapor deposition (CVD). Among them, PVD technology is the most widely used, mainly including: electron beam evaporation, magnetron sputtering, and ion-assisted deposition (IAD), etc. Electron beam evaporation deposits the material by bombarding the target with high-energy electron beams, which has the advantages of high purity and high precision, making it suitable for high-end precision optical components; magnetron sputtering achieves deposition by bombarding the target with plasma ions, which has the advantage of dense film layer and good uniformity, making it suitable for large-scale production; ion-assisted deposition improves the film layer structure by introducing high-energy ions, enhancing the density and stability of the film layer, and can achieve high-quality coating at room temperature, suitable for special substrates such as plastics.

The selection of optical coating materials directly determines the performance of the coating layer. The commonly used materials can be mainly classified into three categories: First, there are dielectric materials, such as silicon dioxide (SiO₂), titanium dioxide (TiO₂), zirconium oxide (ZrO₂), magnesium fluoride (MgF₂), etc. These materials have good light transmittance and adjustable refractive indices, and are the core materials for antireflective films, high-reflection films, and filter films. Among them, magnesium fluoride can increase the transmittance, silicon dioxide has high hardness and good chemical stability, and zirconium oxide has a high refractive index and high-temperature resistance; second, there are metal materials, such as aluminum, silver, gold, etc., which are mainly used to prepare high-reflection films and conductive films. The reflection rate of most metals can reach 78% to 98%, and it can be further increased to over 99% through coating, meeting the requirements of high-end optics; third, there are composite and new materials, such as nanoparticles, quantum dots, chalcogenide glass, etc., which are used to prepare multifunctional composite films and special functional films, adapting to extreme environments and emerging scenarios.

The development history of optical coating technology is a journey from "empirical exploration" to "precise control", from "single function" to "multi-functional integration", and from "lagging behind" to "leading the way". It can be roughly divided into four stages, spanning nearly two centuries, and deeply imprinted with the marks of China's technological development.

In 1835, German chemist Liebig invented the silver mirror reaction, achieving the controlled deposition of metal films for the first time, thus initiating the optical coating technology. During this period, the coating technology mainly relied on manual operations and simple metal film preparation, with limited application scenarios, only used for the production of simple reflectors. In the 1930s, British physicist Ball was the first to prepare a single-layer magnesium fluoride anti-reflection film, increasing the lens transmittance from 80% to 95%, laying the theoretical foundation for optical coating and marking a transition from "empirical operation" to "theoretical guidance" for coating technology. During this stage, China's coating technology was almost nonexistent, and only a few institutions could manually coat simple reflectors, with the core technology monopolized by European and American countries.

With the initial development of optical instruments and laser technology, optical coating technology entered a systematic development stage. The core breakthroughs focused on vacuum coating equipment and multilayer film technology. In 1951, Wang Daheng established the first optical laboratory in New China in Changchun, using simple equipment to develop the first domestically-made anti-reflection film, ending China's history of "having no films available" and opening the path for China's independent exploration of optical coating technology. In 1958, the Changchun Institute of Optics, Fine Mechanics and Physics developed China's first vacuum coating machine, achieving batch production of multilayer dielectric films; in 1965, the Shanghai Institute of Optics and Fine Mechanics developed a high-reflection film for laser nuclear fusion, with a reflection rate of 99.9%, laying the core foundation for the "Shen Guang" device. During this period, internationally, the large-scale production of multilayer films was gradually realized, with the thickness control accuracy of the film layer improving to the nanometer level. The coating process evolved from thermal evaporation to electron beam evaporation and magnetron sputtering, and the application scenarios expanded to laser, aerospace and other fields. However, Chinese technology was still about 20 years behind international levels, mainly relying on imitation and catching up.

With the rapid development of the global optoelectronic industry, optical coating technology has entered a stage of "high precision, large-scale, and diversified" development. China has entered a period of rapid catch-up. During this period, processes such as magnetron sputtering and ion-assisted deposition in the international community have gradually matured, achieving large-area and highly uniform coating, suitable for large-scale scenarios such as photovoltaics and display panels; the design of the film system has evolved from simple periodic structures to complex non-periodic structures, and high-performance film systems such as broadband anti-reflection films and narrow-band filter films have gradually become widespread.

During this period, China achieved several key breakthroughs: Zhejiang University overcame the technology for purifying zirconium dioxide (ZrO₂), reducing the cost of domestic coating materials by 70%; the KAI-400 coating machine developed by the Second Institute of China Electronics Technology Group Corporation has a vacuum degree of 10⁻⁶Pa, approaching the advanced international level; Fuyao Glass introduced the Low-E coating technology and then absorbed and utilized it, formulating China's building glass coating standards, breaking the foreign monopoly. In 2008, the membrane structure of the "Bird's Nest" for the Beijing Olympics used domestic PTFE coating, with a durability of 30 years, demonstrating China's catching-up speed in coating technology to the world. At the same time, China gradually achieved the domestic substitution of coating equipment and materials, reducing its reliance on imports, and its applications covered consumer electronics, construction, laser and other fields, gradually narrowing the gap with international advanced levels.

In the early 2010s of the 21st century, with the breakthroughs in advanced optoelectronic technology, quantum technology, and aerospace technology, optical coating technology entered a new stage of "atomic-level precision, multi-functional integration, and global competition". China gradually achieved the transformation from "following" to "leading". Internationally, advanced processes such as atomic layer deposition (ALD) and high-power pulsed magnetron sputtering (HiPIMS) have gradually matured, and the control accuracy of the coating layer has been improved to the sub-nanometer level. New coating technologies such as metamaterials and smart-responsive coatings have gradually entered the application stage.

China has achieved multiple breakthroughs in the high-end field: The 40-layer Mo/Si film developed by the Institute of Optics and Electronics of the Chinese Academy of Sciences has a reflection rate of 98.5%, which supports the manufacturing of 28nm domestic chips; Crystal Optoelectronics' 905nm narrowband film occupies 70% of the global market, reducing the cost of intelligent vehicles by 60%; The single photon polarization-preserving film developed by the University of Science and Technology of China has a quantum state transmission loss of less than 0.1%, helping the "Moxi" satellite achieve a 1,000-kilometer entanglement distribution. In 2023, China led the formulation of the "International Standard for Nano Multilayer Optical Coating Technology" (ISO 23618), achieving a transformation from a "follower" to a "standard setter", marking that China's optical coating technology has entered the international advanced ranks. Currently, the annual export volume of coating equipment in China is increasing by 25%, and the technology is reverse-licensed to Europe and the United States, forming a complete industrial chain. Its market share in fields such as consumer electronics, automotive optics, and quantum communication is gradually increasing.

With the rapid development of emerging fields such as AI, quantum technology, AR/VR, and autonomous driving, optical coating technology is evolving from "passive optical functional layers" to "intelligent, precise, green, and multi-functional integrated" active photonic control systems. The core trends can be summarized into six directions, taking into account both technological upgrading and scenario expansion, to drive the photonic industry to a higher level of development.

AI and digital technologies are redefining the entire process of optical coating, significantly enhancing design efficiency, accuracy and yield, and becoming the core driving force for future development. Traditional multilayer film design relies on experience and iterative processes, which are time-consuming and difficult to achieve global optimality. However, AI models (such as OptoGPT) can quickly traverse a vast number of material combinations and film thickness parameters, completing the traditional months-long optimization process within just a few hours. This enables the design of complex multibandwidth, low-loss, and high-damage-threshold coating systems. Currently, the proportion of non-periodic coating systems has increased to 67%, and the broadband expansion capability has improved by more than 40% compared to traditional structures.

At the same time, by integrating machine learning with online monitoring technologies (such as optical monitoring, mass spectrometry, and ellipsometry), real-time feedback and adaptive adjustments of the deposition process are achieved, enabling the control accuracy of film thickness to be pushed from the nanometer level to the sub-nanometer level. The yield of high-end filters has significantly increased from 75% to 96.5%, and the single-piece production cycle has been shortened by 30%. The application of virtual simulation and digital twin technology can predict the stress, adhesion, and environmental stability of the film layer in advance, reducing the cost of trial and error, accelerating the transformation from research and development to mass production, and promoting the transition of optical coating from "manufacturing" to "intelligent manufacturing".

The traditional PVD process has undergone continuous iterations, and technologies such as atomic layer deposition (ALD), high-power pulsed magnetron sputtering (HiPIMS), and ion-assisted deposition (IAD) have become the mainstream in high-end manufacturing, achieving breakthroughs in "atomic-level controllability, large-area uniformity, and low defect rates". Atomic layer deposition (ALD) precisely grows single atomic layers, with thickness control reaching the 0.1nm level, and density approaching the theoretical value. It is suitable for scenarios with zero tolerance for defects such as ultra-precision optics, quantum devices, and biosensing, and is expected to occupy 35% of the high-end semiconductor detection optical component market share by 2026.

High-power pulsed magnetron sputtering (HiPIMS) increases the kinetic energy of the sputtered particles by 10 to 100 times, resulting in dense, highly adhesive, and controllable stress films. It offers both high purity and high production capacity, gradually narrowing the performance gap compared to electron beam evaporation. Ion-assisted deposition (IAD) significantly enhances the density, hardness, and environmental stability of the film by introducing high-energy ions. The anti-reflection films treated with IAD show a center wavelength drift of less than 1 nm after aging for 1000 hours at 85°C/85% RH, making them a standard process for laser optics and infrared windows.