multicoatings

Multicoatings, also called multilayer coatings, are thin-film stacks composed of alternating layers of materials with different refractive indices deposited on a substrate. By engineering thickness and material choice, interference effects are used to tailor light transmission, reflection, absorption, or other properties, while offering protection or added functionality.

Dielectric multilayer coatings are widely used to control reflectance and transmittance in optical systems. Antireflection coatings reduce surface reflections on lenses and displays; high-reflectance coatings are used for mirrors; broadband stacks can provide more uniform performance over wide spectral regions. Multilayer designs can be customized for specific bands, angular performance, and environmental conditions.

Manufacturing and design: The layers are deposited by vacuum techniques such as physical vapor deposition (PVD), including sputtering and evaporation, chemical vapor deposition (CVD), or atomic layer deposition (ALD). Control of thickness to within a fraction of a wavelength is critical, often achieved with optical monitors or in-situ interferometry. Design principles often employ quarter-wave stacks and matrix methods to predict spectral response; more complex optimizations enable broadband or spectrally selective performance.

Materials: Dielectric layers such as SiO2, Al2O3, MgF2 (low index), TiO2, Ta2O5, HfO2, ZrO2 (high index). Metal layers like aluminum or silver are used in some high-reflectance or protected-mirror designs, though they can introduce losses or environmental sensitivity.

Performance and testing: Performance is evaluated by reflectance, transmittance, and absorption spectra, as well as environmental durability. Quality control includes spectrophotometry, ellipsometry, scratch and adhesion testing, and environmental aging.

Applications: ophthalmic and camera optics, telescope and laser systems, solar panels, architectural glazing, and protective coatings in harsh environments.

Challenges include managing residual stress, cracking, delamination, coating defects, and aging or moisture sensitivity.