Types Anti-reflective coating

1 types

1.1 index-matching
1.2 single-layer interference
1.3 multi-layer interference
1.4 absorbing
1.5 moth eye
1.6 circular polarizer

types
index-matching

the simplest form of anti-reflective coating discovered lord rayleigh in 1886. optical glass available @ time tended develop tarnish on surface age, due chemical reactions environment. rayleigh tested old, tarnished pieces of glass, , found surprise transmitted more light new, clean pieces. tarnish replaces air-glass interface 2 interfaces: air-tarnish interface , tarnish-glass interface. because tarnish has refractive index between of glass , air, each of these interfaces exhibits less reflection air-glass interface did. in fact, total of 2 reflections less of naked air-glass interface, can calculated fresnel equations.

one approach use graded-index (grin) anti-reflective coatings, is, ones continuously varying index of refraction. these, possible curtail reflection broad band of frequencies , incidence angles.

single-layer interference

the simplest interference anti-reflective coating consists of single thin layer of transparent material refractive index equal square root of substrate s refractive index. in air, such coating theoretically gives 0 reflectance light wavelength (in coating) equal 4 times coating s thickness. reflectance decreased wavelengths in broad band around center. layer of thickness equal quarter of design wavelength called quarter-wave layer .

the common type of optical glass crown glass, has index of refraction of 1.52. optimal single-layer coating have made of material index of 1.23. unfortunately, there no solid materials such low refractive index. closest materials physical properties coating magnesium fluoride, mgf2 (with index of 1.38), , fluoropolymers (which can have indices low 1.30, more difficult apply). mgf2 on crown glass surface gives reflectance of 1%, compared 4% bare glass. mgf2 coatings perform better on higher-index glasses, index of refraction close 1.9. mgf2 coatings commonly used because cheap, , when designed wavelength in middle of visible band, give reasonably anti-reflection on entire band. researchers have produced films of mesoporous silica nanoparticles refractive indices low 1.12, function antireflection coatings.

multi-layer interference

by using alternating layers of low-index material silica , higher-index material, possible obtain reflectivities low 0.1% @ single wavelength. coatings give low reflectivity on broad band of frequencies can made, although these complex , relatively expensive. optical coatings can made special characteristics, such near-zero reflectance @ multiple wavelengths, or optimal performance @ angles of incidence other 0°.

absorbing

an additional category of anti-reflection coatings so-called absorbing arc . these coatings useful in situations high transmission through surface unimportant or undesirable, low reflectivity required. can produce low reflectance few layers, , can produced more cheaply, or @ greater scale, standard non-absorbing ar coatings. (see, example, patent 5,091,244.) absorbing arcs make use of unusual optical properties exhibited in compound thin films produced sputter deposition. example, titanium nitride , niobium nitride used in absorbing arcs. these can useful in applications requiring contrast enhancement or replacement tinted glass (for example, in crt display).

moth eye

moths eyes have unusual property: surfaces covered natural nanostructured film, eliminates reflections. allows moth see in dark, without reflections give location away predators. structure consists of hexagonal pattern of bumps, each 200 nm high , spaced on 300 nm centers. kind of antireflective coating works because bumps smaller wavelength of visible light, light sees surface having continuous refractive index gradient between air , medium, decreases reflection removing air-lens interface. practical anti-reflective films have been made humans using effect; form of biomimicry.

such structures used in photonic devices, example, moth-eye structures grown tungsten oxide , iron oxide can used photoelectrodes splitting water produce hydrogen. structure consists of tungsten oxide spheroids of several 100 micrometer size coated few nanometers thin iron-oxide layer.

circular polarizer

reflections blocked circular polarizer

a circular polarizer laminated surface can used eliminate reflections. polarizer transmits light 1 chirality ( handedness ) of circular polarization. light reflected surface after polarizer transformed opposite handedness . light cannot pass through circular polarizer because chirality has changed (e.g. right circular polarized left circularly polarized). disadvantage of method if input light unpolarized, transmission through assembly less 50%.