Application and Classification of Optical Thin Films

Optical thin films are functional films that work based on the interaction of light with thin films. They can achieve functions such as spectral transmission, spectral reflection, spectral absorption, anti-reflection, reflection enhancement, beam splitting, high-pass filtering, low-pass filtering, narrow-band filtering, and more, in terms of changing light intensity. There are many types of optical thin films, and these films provide various performance capabilities to optical components, playing an important or even decisive role in realizing the functions of optical instruments and influencing the quality of optical instruments.

Traditional optical thin films are an important part of modern optical instruments and various optical devices. By depositing one or more layers of thin films on the surface of various optical materials, the intensity, polarization state, and phase change of transmitted or reflected light can be altered using the interference effect of light. Thin films can be deposited on various materials such as optical glass, plastic, fiber optic, and crystals. The thickness of thin films can range from a few nanometers to tens or hundreds of micrometers. Optical thin films can provide good durability, optical stability, low cost, and almost no increase in the volume and weight of materials. Therefore, they are the preferred method for changing the optical parameters of a system. It can even be said that modern optical instruments and various optical devices would not exist without optical thin films. Over the course of more than two hundred years of development, optical thin films have formed a complete optical theory - thin film optics. Optical thin films are widely used in various optical devices (such as laser resonators, interference filters, optical lenses, etc.), and their important role in the optoelectronic field is gradually recognized.

1. Interference Filters

Devices that divide a beam of light into two parts are called beam splitters. The working part of a beam splitter is usually a coated surface, which has specific reflectance and transmittance in a certain wavelength range. Usually, this surface is inclined, so the incident light and reflected light are separated. The predetermined reflectance and transmittance values of beam splitters vary depending on their application.

Different beam splitters often have different transmittance and reflectance ratios, known as splitting ratios (T/R). The most commonly used is the neutral beam splitter, with T/R = 50/50, which splits a beam of light into two beams with the same spectral composition. Because it has the same transmittance and reflectance ratio for all wavelengths in a certain wavelength range, the reflected and transmitted light does not have any color and appears neutral. There are two common structures for neutral beam splitters: one is a coated film on a transparent flat substrate, and the other is a film-coated on two right-angle prisms, which are then bonded together face to face to form a cube. The commonly used beam splitters are divided into two categories: metal beam splitters and dielectric beam splitters. Metal beam splitters have a wider spectral width but higher absorption loss and lower splitting efficiency. Dielectric beam splitters have high splitting efficiency, obvious polarization effects, and significant spectral dispersion. Compared to metal beam splitters, dielectric beam splitters have lower absorption rates, resulting in higher splitting efficiency, which is the advantage of dielectric beam splitters. However, dielectric beam splitters are more sensitive to wavelength, making it challenging to achieve neutral splitting. Moreover, dielectric beam splitters generally have significant polarization effects, which is a limitation.

2. Anti-reflective Film (also known as AR film or anti-reflection film)

Assuming that light is vertically incident on the surface, the intensity of the reflected light on the surface, compared to the intensity of the incident light (reflectance), is only determined by the ratio of the refractive indices of the adjacent media.

The reflectance of each surface of crown glass with a refractive index of 1.52 is approximately 4.2%. Glass with a higher refractive index, such as flint glass, exhibits more significant surface reflection. This surface reflection has two serious consequences: loss of light energy, resulting in decreased brightness of the image; and the surface-reflected light undergoes multiple reflections or diffusions, with a portion becoming stray light that eventually reaches the image plane, reducing the contrast of the image. Complex systems such as television and movie camera lenses consist of many surfaces adjacent to air, and without the application of anti-reflective coating, their performance would be greatly diminished.

Anti-reflective coatings are extensively used in optical instruments in the visible spectrum region, comprising the majority of the production of anti-reflection films. Almost all optical devices require anti-reflection treatment.

3. Interference Cutoff Filters

Interference cutoff filters, also known as wavelength-selective filters, are widely used to transmit a specific wavelength range of light with high transmittance while abruptly changing the transmittance to high reflectance (or suppression) for wavelengths outside this range. Filters that suppress the short wavelength region and transmit the long wavelength region are called long-pass filters, while filters that suppress the long wavelength region and transmit the short wavelength region are called short-pass filters. In most cases, it is desired to cutoff all light shorter or longer than a specific wavelength. The usual method is to combine interference filters with absorbing filters. They can be used both as short-pass filters for long-wavelength cutoff and as long-pass filters for short-wavelength cutoff. The cutoff position can be moved arbitrarily by changing the wavelength of the monitoring film thickness.

4. Reflective Films

Reflective films are optical components used to reflect most or almost all of the incident light energy. In some optical systems, high reflectivity is required, such as in laser mirrors, where a reflectance of 90% or more for a certain frequency of monochromatic light is required. To enhance the reflectivity, a layer of high reflectance transparent film is often deposited on the glass surface, utilizing the interference phase condition satisfied by the optical path difference of the reflected light between the upper and lower surfaces, thus increasing the reflectivity. Metal films have high reflectance but also higher absorption, while dielectric films have not only high reflectance but also lower absorption. Aluminum is the only material that has high reflectance from ultraviolet to infrared (0.2-30 μm). There is a minimum reflectance value of approximately 86% at around 0.85 μm. Aluminum films have strong adhesion to substrates, better mechanical strength and chemical stability, making them widely used as reflective films. When a newly deposited aluminum film is exposed to atmospheric conditions at room temperature, a layer of amorphous transparent Al2O3 film is immediately formed on the surface, and the oxide rapidly grows to approximately 15-20 nm within a short time, followed by slow growth, reaching around 50 nm after a month. The presence of oxide decreases the reflectance of the aluminum film, especially in the region below 200 nm. Therefore, a protective layer of MgF2 film is used. In the visible light range, SiO is usually used as the initial material, evaporated to form a silicon oxide film as a protective film for the aluminum film. The optimal conditions for preparing aluminum films are: high-purity aluminum (99.99%); rapid evaporation in high vacuum (50-100 nm/s); substrate temperature below 50°C.

Silver films have the highest reflectivity among all known materials in the visible and infrared regions. In the visible light region and the infrared region, the reflectivity reaches about 95% and 99%, respectively. However, silver films have poor adhesion, mechanical strength, and chemical stability, so they are mainly used for short-term applications. The reflectance of silver decreases in the ultraviolet region, starting to decline at around 400 nm and reaching about 4% near 320 nm. When a silver film is exposed to the air, the reflectance gradually decreases due to the formation of silver oxides (AgO, Ag2O3) and silver sulfides on the surface. Therefore, a protective film is plated on the silver film. The optimal preparation process is similar to that of aluminum, involving high vacuum, rapid evaporation, and low substrate temperature.

One important factor in reducing film reflectivity is scattering. There are various causes of scattering losses, arising from the nucleation and growth mechanisms of the film resulting in an uneven microstructure, leading to scattering. Observing the microstructure of the cross-section of multilayer films using an electron microscope shows a clear columnar structure, with the film interior filled with voids and the surface becoming uneven. In addition, the roughness and defects on the substrate surface, as well as particles sputtered from the evaporation source, dust in the film, cracks, and pinholes, all contribute to a complex scattering model. Overall, scattering can be divided into two types: volume scattering and surface scattering.