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 similar to the coherence length; for visible light it is most often observed between 200 and 1000 nm of thickness. Layers at this scale can have remarkable reflective properties due to light wave interference and the difference in

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...

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 Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an ...

, is observable in soap bubbles and oil slicks. More general periodic structures, not limited to planar layers, exhibit

structural coloration Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination wi ...

with more complex dependence on angle, and are known as

photonic crystal A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the ato ...

s. In manufacturing,

thin film A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many a ...

layers can be achieved through the deposition of one or more thin layers of material onto a substrate (usually

glass Glass is a non-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...

). This is most often done using a

physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and poly ...

process, such as evaporation or sputter deposition, or a chemical process such as

chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (subst ...

. Thin films are used to create optical coatings. Examples include low emissivity panes of

for houses and cars, anti-reflective coatings on

glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples or ...

, reflective baffles on car headlights, and for high precision optical filters and

mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...

s. Another application of these coatings is spatial filtering.

Examples in the natural world

Peacock butterfly (inachis io) 2.jpg, The blue wing patches of the '' Aglais io''. Common Bluebottle-Graphium sarpedon teredon.JPG, ''

Graphium sarpedon ''Graphium sarpedon'', the common bluebottle or blue triangle in Australia, is a species of swallowtail butterfly that is found in South and Southeast Asia, as well as eastern Australia. There are approximately sixteen subspecies with differi ...

''. Parotia lawesii by Bowdler Sharpe.jpg, The breast feathers of the ''

Lawes's parotia Lawes's parotia (''Parotia lawesii''), is a medium-sized (up to 27 cm long) passerine of the bird-of-paradise family, Paradisaeidae. It is distributed and endemic to mountain forests of southeast and eastern Papua New Guinea. Occasional ...

.'' Tachinidae.jpg, The

that can be seen on many

insect wing Insect wings are adult outgrowths of the insect exoskeleton that enable insects to fly. They are found on the second and third thoracic segments (the mesothorax and metathorax), and the two pairs are often referred to as the forewings and hin ...

s is due to thin-film optics. Ranunculus macro.jpg, The glossy flowers of '' Ranunculus'' buttercups.Buttercups focus light to heat their flowers and attract insects
''New Scientist'' 25 February 2017 Thin-film layers are common in the natural world. Their effects produce colors seen in soap bubbles and oil slicks, as well as the

of some animals. The wings of many insects act as thin-films, because of their minimal thickness. This is clearly visible in the wings of many flies and wasps. In butterflies, the thin-film optics is visible when wing itself is not covered by wing scales, which is the case in the blue wing spots of the '' Aglais io'' and the blue-green patches of the ''

''. In

buttercup ''Ranunculus'' is a large genus of about almost 1700 to more than 1800 species of flowering plants in the family Ranunculaceae. Members of the genus are known as buttercups, spearworts and water crowfoots. The genus is distributed in Europe ...

s, the flower's gloss is due to a thin-film, which enhances the flower's visibility to pollinating insects and aids in temperature regulation of the plant's reproductive organs.

Examples in the natural world

See also

References

Further reading