Thin-film optics is the branch of

optic Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

s 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 In physics, coherence length is the propagation distance over which a coherent wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves differ ...

; 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 Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...

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, or ...

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 bubble A soap bubble is an extremely thin film of soap or detergent and water enclosing air that forms a hollow sphere with an iridescent surface. Soap bubbles usually last for only a few seconds before bursting, either on their own or on contact wi ...

s and oil slicks. More general periodic structures, not limited to planar layers, exhibit structural coloration with more complex dependence on angle, and are known as photonic crystals. 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 ap ...

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

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

). This is most often done using a physical vapor deposition process, such as evaporation or

sputter deposition Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re ...

, 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 (substra ...

. Thin films are used to create

optical coating An optical coating is one or more thin layers of material deposited on an optical component such as a lens, prism or mirror, which alters the way in which the optic reflects and transmits light. These coatings have become a key technology in th ...

s. Examples include

low emissivity Low emissivity (low ''e'' or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect, and emit radiant energy according to Planck's law but here, the primary conc ...

panes of

for houses and cars,

anti-reflective coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effic ...

s 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 Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviol ...

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 filter A spatial filter is an optical device which uses the principles of Fourier optics to alter the structure of a beam of light or other electromagnetic radiation, typically coherent laser light. Spatial filtering is commonly used to "clean up" the ou ...

ing.

Examples in the natural world

Peacock butterfly (inachis io) 2.jpg, The blue wing patches of the ''

Aglais io ''Aglais io'', the European peacock, more commonly known simply as the peacock butterfly, is a colourful butterfly, found in Europe and temperate Asia as far east as Japan. It was formerly classified as the only member of the genus ''Inachis'' ( ...

''. 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 diffe ...

''. 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. Occasionally, th ...

.'' Tachinidae.jpg, The thin-film interference that can be seen on many insect wings is due to thin-film optics. Ranunculus macro.jpg, The glossy flowers of ''

Ranunculus ''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 ...

'' 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 structural coloration 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 ''

'' 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 Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds, a ...

insects and aids in temperature regulation of the plant's reproductive organs.

Examples in the natural world

See also

References

Further reading