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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, ultravio ...
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 dif ...
; 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 extr ...
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 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 atomic ...
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 ...
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 ...
). 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 polym ...
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 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 con ...
panes of
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 ...
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 ...
, reflective baffles on car headlights, and for high precision optical filters and
mirror A mirror or looking glass is an object that 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 direction of the im ...
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 o ...
ing.


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''. Parotia lawesii by Bowdler Sharpe.jpg, The breast feathers of the '' Lawes's parotia.'' Tachinidae.jpg, The
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 ...
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 hindw ...
s 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 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 ...
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 '' Graphium sarpedon''. 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.


See also

*
Dichroic filter A dichroic filter, thin-film filter, or interference filter is a color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characteri ...
*
Dichroic prism A dichroic prism is a prism that splits light into two beams of differing wavelength ( colour). A trichroic prism assembly combines two dichroic prisms to split an image into 3 colours, typically as red, green and blue of the RGB colour model. T ...
* Dielectric mirror *
Dual polarisation interferometry Dual-polarization interferometry (DPI) is an analytical technique that probes molecular layers adsorbed to the surface of a waveguide using the evanescent wave of a laser beam. It is used to measure the conformational change in proteins, or othe ...
*
Fresnel equations The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media. They were deduced by Augustin-Jean Fres ...
*
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 ...
*
Transparent materials In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions ...


References


Further reading

* An excellent introduction to thin-film optics, with a focus on biology. Cites more rigorous treatments. * Z. Knittl: ''Optics of thin films'', Wiley, 1981. * D.G. Stavenga,
Thin film and multilayer optics cause structural colors of many insects and birds
Materials Today: Proceedings 1S, 109 – 121 (2014). * * {{Authority control