Thin-film optics is the branch of optics that deals with very thin structured layers of different materials. In order to exhibit thin-film optics, the thickness of the layers of material must be on the order of the wavelengths of visible light (about 500 nm). Layers at this scale can have remarkable reflective properties due to light wave interference and the difference in refractive index between the layers, the air, and the substrate. These effects alter the way the optic reflects and transmits light. This effect, known as thin-film interference, is observable in soap bubbles and oil slicks.
More general periodic structures, not limited to planar layers, are known as photonic crystals.
In manufacturing, thin film layers can be achieved through the deposition of one or more thin layers of material onto a substrate (usually glass). This is most often done using a physical vapor deposition process, such as evaporation or sputter deposition, or a chemical process such as chemical vapor deposition.
Thin films are used to create optical coatings. Examples include low emissivity panes of glass for houses and cars, anti-reflective coatings on glasses, reflective baffles on car headlights, and for high precision optical filters and mirrors. Another application of these coatings is spatial filtering.[1]
Thin-film layers are common in the natural world. Their effects produce colors seen in soap bubbles and oil slicks, as well as in some animals. For example, the light collecting tapetum lucidum of many nocturnal species and the photophores of bioluminescent squid (e.g., the Bobtail squid). In many cases, iridescent colors that were once thought to result from planar layers, such as in opals, peacocks, and the Blue Morpho butterfly, turn out to result from more complex periodic photonic-crystal structures.