Exploring the Physics Behind the Optical Spectra of Thin Films

Applied Tech Review | Monday, August 01, 2022

The physics in thin film optical spectra are frequently necessary to alter the specular and transmission properties of mirror-like characteristics governed by the laws of reflection and refraction of these components

FREMONT, CA: The majority of optical or optoelectronic systems contain optical parts with surfaces and forms specifically created for the best interaction with light, including lenses, mirrors, gratings, detectors, and others. To improve the performance of optical systems, it is frequently necessary to alter the specular and transmission properties mirror-like characteristics, governed by the laws of reflection and refraction of these components. These properties are determined by the optical properties of the material and surrounding medium.

An optical component's transmission, reflection, or polarisation qualities can be improved via optical thin-film coatings. For instance, the surface of an uncoated glass component will reflect over four per cent of the incident light. Each air-glass interface's reflection can be brought down to less than 0.1 per cent with an anti-reflection coating. Mirror surfaces could have their reflectivity increased to over 99.99 per cent by applying a highly reflective dielectric coating. Typically, tiny layers of materials like oxides, metals, or rare earth elements are combined to form an optical coating.

The number of individual layers–their thickness and doping, as well as the variations in the refractive indices of the layers–have an impact on how well a thin film optical coating performs. Due to interference effects, the desired improvement of the optical characteristics is achieved by varying the refractive indices of the layers and varying the thickness of the individual coating layers, which can range from a few nanometers to several hundred nanometers. Since the coating is typically on the component's exterior, a thin layer is frequently anticipated to serve additional purposes in addition to its primary one, such as reducing corrosion and boosting abrasion resistance.

The majority of thin-film optical coatings are made to improve an optical component's performance over a range of wavelengths, at a certain angle of incidence, and for a particular polarisation of light such as linear polarization, elliptical polarization, or random polarization. A coating's performance will be noticeably reduced or even lose its entire optical function if it is used in a spectral range, angle of incidence, or polarisation other than those for which it was intended.

By using a variety of chemical vapour deposition (CVD) or physical vapour deposition (PVD) processes, a planned sequence of materials is condensed onto the surface of the optical component to create thin-film optical coatings. Several PVD techniques, such as ion-assisted electron-beam evaporative deposition, ion beam sputtering, advanced plasma deposition, and plasma-assisted reactive magnetron sputtering, are frequently employed to apply optical coatings.

Anti-reflection coatings on various optical components are the simplest yet most common use of thin optical films. Researchers significantly reduced the amount of unwanted reflected light in optical equipment such as camera lenses, microscope objectives, binoculars, and spectacle lenses by investigating the physics of low refractive index coatings put over high refractive index optical material. Such anti-reflective coatings are quite beneficial for modern high refractive index plastic lenses since they lessen glare, especially when driving at night.

Magnesium fluoride thin films with a thickness of around a quarter wavelength are the foundation of anti-reflective coatings, which lower the reflectance of the coated component. Greater performance across the full visible spectrum is needed for more demanding applications, though 400 nm to 700 nm. The complexity of the coating's structure increases with the size of the needed spectrum for reflection reduction. To cover a considerably wider spectral range, several multilayer coatings made of layers of tantalum oxide, aluminium oxide, and magnesium fluoride have been created.

In reality, the current optical apparatus is frequently required to function throughout a much wider spectrum that ranges from UV to long wavelengths (IR). Different coating materials are needed for optical components and devices that function in numerous spectral areas, particularly at long wavelengths in the infrared spectrum, including communications equipment, satellite imagery cameras, ground-and space-based telescopes, and many more. For anti-reflective thin-film coatings suitable for the short wave IR and mid-wave IR regions wavelengths of 0.9-1.7 m and 3-5 m. Respectively, oxide compounds with low, medium, and high refractive indices, such as silicon oxide, aluminium oxide, and yttrium oxide, can be used. These compounds have excellent optical properties at wavelengths shorter than 7 m. The best performing coating material is a mixture of fluoride-based compounds, group IIB-VIA compounds (ZnS and ZnSe), and germanium.

Many pieces of large-aperture optical equipment, including astronomical observatories, high-power laser systems, and space-based optics working at IR wavelengths, now need the use of silver-based high-performance reflective coatings. Silver mirror performance and endurance have significantly increased thanks to multi-layer thin films that combine protective layers of silicon nitride, nickel-chromium nitride, and highly reflective silver film. Examples include the eight-meter primary mirrors of the telescopes at the Gemini Observatory in Hawaii, which are coated to work at their peak efficiency.


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