Anti-reflective surfaces

H. Hattori Anti-Reflection Surface with Particle Coating Deposited by Electrostatic Attraction , Adv. Mater. 2001,13(1), 51-54. 

The ingenious nocturnal moth evades its predators with the aid of an anti-reflection surface in the back of its eyes, which appear black, rather than glowing like the eyes of other insects, when a light shines on them. The moth achieves this by dint of a regular array of small conical protuberances, about 200 nm (two ten-thousandths of a millimetre) in size, with a refractive index that changes from tip to base. The reflection of light from glass, say. 

If we now consider applications in which the anti-reflective surfaces are designed for visible wavelengths or for solar radiation by using the above mentioned design rules, we get the results in Table 1 for a substrate material with refractive index Hs = 1.5 ... 

Get the light into the detector. This may sound trivial, but unless a good anti-reflection coating is used, a significant fraction of the light will be reflected at the surface of the photosensitive material. 

It s quite common when driving at night to be dazzled by the lights of the vehicle behind as they reflect from the driver s new-view or door mirror. We can prevent the dazzle by forming a layer of coloured material over the reflecting surface within an electrochromic mirror. Such mirrors are sometimes called smart mirrors or electronic anti-dazzle mirrors . 

The crosslinkable COCs have been suggested for the use as alternatives for a glass substrate of a liquid crystal display devices and an electroluminescence (EL) display devices. Moreover, a crosslinkable COC can be used as a polarizing film, surface protective film, retardation film, transparent conductive film, light diffusion film, film for EL display devices, transparent conductive composite material, anti-reflection film, etc. Methods how to prepare these types of films have been described in detail (47). 

Optics Issues relative to hygroscopic optics and the need to pay attention to mirror mounts relative to vibration and/or thermal effects have already been addressed. Zinc selenide is an important alternative material, especially when anti-reflection coated. If potassium bromide absolutely has to be used for its lower transmission range (down to 400 cm-1) then a protective coating must be used. Most process analyzers use protective windows between the spectrometer and the sample interface. If used, back reflections from the window surfaces into the interferometer must be avoided because these will cause photometric errors. 

A substrate 10 of HgCdTe is provided with an upper surface region 11 formed by an annealing procedure, or as an epitaxial layer or evaporated film. A layer of insulating material 12 is formed in which windows are provided. The windows are partially filled with a thin layer of metal 13 which is deposited therein to form a metal-semiconductor diode with the upper surface region. The metal layer is deposited to a thickness on the order of 10-50 nm thick and is sufficiently thin to be semi-transparent to infrared radiation. A thick layer of metal 14 is deposited to form an expanded contact and an anti-reflection coating 15 is provided. External conductors in the form of jumper wires 16 are ball bonded to the contact 14. 

Also surface optical properties of a material sometimes need to be changed, for example in making anti-reflection coating for lenses or reflective surfaces for CDs, the magnetic properties may need to be influenced as in the case of giving a ferroelectric surface to a plastic for magnetic recording, and, perhaps most extensively of all, the surface electrical properties need to be controlled in microelectronic devices used in computers and all modern electronic equipment. 

A technique for thin-fitm preparation at low temperatures has been developed in which photoirradiation is combined with the sol-gel method. This has been used to obtain Si02 thin films and surface coatings for display. Harder, denser films were prepared at lower temperatures than by conventional heat treatment. Anti-reflection/antistatic thin films prepared by this new process have low surface... 

Adhesion is a more difficult problem. There are many chemically different interfaces in a laminant stack glass-EVA, EVA-solar cell surface (oxidized silicon or some anti reflection (AR) coating), cell back surface metallization-EVA, EVA-back sheet, and EVA-bus bars (copper or tinned copper). Each of these interfaces is important, because mechanically good adhesive bonds will often fail by delamination after exposure to humidity in the field. Water vapor will permeate through polymers and, if there is a non-chemically bonded surface, water may collect and cause failure by displacement. Tables I, II, and III provide details of some of the research efforts in adhesion. 

Anti reflection Coating - A thin coating of a material applied to a photovoltaic cell surface that reduces the light reflection and increases light transmission. 

A window can absorb heat from the laser beam by intrinsic bulk absorption or at the surfaces (through intrinsic surface absorption mechanisms or at the anti-reflection (AR) coatings). The absorbed heat induces radial and axial temperature gradients which in turn result in two distinct forms of optical distortion ... 

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