Thin film coatings

Thin Aims are dielectric or metallic materials deposited on an optical surface. Their thickness is comparable to, or less than, the wavelength of the light for which they are used. When a beam of light is incident 

The optical thickness Dopt of any optical element is defined as the equivalent vacuum thickness (i.e. the distance that light would travel in vacuum in the same amount of time as it takes to traverse the optical element). This is given by 

As just stated, two requirements need to be fulfilled for exact cancellation of reflected beams by a single-layer coating, i.e. the two reflections have to be out of phase by exactly tt, and exactly the same intensity is required. Consequently, a single-layer AR coating must be an odd number of quarter wavelengths to achieve the correct phase relation for wave cancellation. In addition, the refractive index of the thin-film medium has to obey the relation Mair TF S- 

The angle of incidence of radiation on the thin-film coating has one major effect on the reflectivity, because the path difference of the front and rear surface reflection from any layer is a function of angle. The change in path difference results in a change of phase difference between the two interfering reflections, and thus perfect destructive interference is not any longer maintained. 

Because of these dependencies, in particular the angular parameter, it is rather a complex task to evaluate thin-film performance exactly as stated above, for simplicity the discussion here will be restricted to perpendicular incidence of the light beam onto coated optical surfaces. 

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Nonmilitary infrared apphcations for germanium include CO2 lasers (qv), intmsion alarms, and pohce and border patrol surveillance devices. Germanium is used as a thin-film coating for infrared materials to decrease reflection losses or to provide heavy filtering action below 2 p.m. 

In the electronics industry, gold is used as fine wires or thin film coatings and frequendy in the form of alloys to economize on gold consumption and to impart properties such as hardness. Gold has properties that satisfy specific requirements not achievable with less expensive metals (see Electrical connectors Electronics coatings Thin films). 

As with ah thin-film PV technologies, the projected manufacturing costs of a-Si H ahoy PV modules fah rapidly with annual manufacturing volume, ie, MWp /yr. The primary driver of this volume cost reduction is the volume—cost relationship of commercially available thin-film processing equipment. Thin-film coating machines often have capacities equivalent to 3—5 yr, so that manufacturing economies of scale are more fully realized at the... 

Polyacetylenes. The first report of the synthesis of a strong, flexible, free-standing film of the simplest conjugated polymer, polyacetylene [26571-64-2] (CH), was made in 1974 (16). The process, known as the Shirakawa technique, involves polymerization of acetylene on a thin-film coating of a heterogeneous Ziegler-Natta initiator system in a glass reactor, as shown in equation 1. 

Fig. 19. Schematic representation of a typical sol-gel thin film coating. Diagram adapted from Ref. 95. ...

Schicht, /. layer ply stratum, bed. course (when thin) film, coat lamina Ph< og.) emulsion charge (of a furnace) batch shift, turn (of work), -boden, m. (Metal.) mixing place, -dicke, /. thickness of a layer or stratum, -ebene, /. bedding plane. 

Hz for thick coatings such as reinforced coal tar enamel, being selected to minimise interference from commonly occurring frequencies while maximising the distance the signal will travel, some 5-10 km on a reasonably well-coated pipeline. For thin film, coatings, such as a fusion-bonded epoxy, a frequency of 200 Hz has been found more appropriate. 

For recording applications, new approaches to high-quality polymeric film substrates are needed. Improved automation and control of thin-film coating are also important. 

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

GP 1] [R 10] By proper heater design, membrane-based reactors with internal heaters allowed one to reach quasi-uniform temperatures at the membrane, which determines the catalyst temperature [19]. This thermal uniformity was checked during reaction, i.e. when large heats were released in the oxidation of ammonia and needed to be transferred out of the reaction zone (Figure 3.29). Thin-film-coated temperature sensors in the center and at the edges of the membrane served to monitor the lateral temperature difference. 

The particular colors that are observed at different angles will depend critically on the thickness of the thin film coating. Precision instrumentation is required to carefully control film thickness during production. The magnitude of the optical effect depends on the density of flakes in the ink, while the quality of the optical effect depends on the precise orientation or alignment of these flakes with respect to the paper surface. 

The pH measurement can by realized using sol-gel films and evanescent-wave sensors method74. To incorporate a near-infrared pH sensitive fluorescent dye, a thin-film coating on the core of a multimode fiber was used. By evanescent wave excitation an absorption or fluorescent based sensor can be realized for use in high pH regions. 

S. Lu, K. Wu, X. Dang, and S. Hu, Electrochemical reduction and voltammetric determination of metronidazole at a nanomaterial thin film coated glassy carbon electrode. Talanta 63, 653-657... 

Zhang, J. Tang, X. Dong, J. Wei, T. Xiao, H., Zeolite thin film coated long period fiber grating sensor for measuring trace chemical, Opt. Express 2008, 16, 8317 8323... 

Pilla, P. Foglia Manzillo, P. Giordano, M. Korwin Pawlowski, M. L. Bock, W. J. Cusano, A., Spectral behavior of thin film coated cascaded tapered long period gratings in multiple configurations, Opt. Express 2008, 16, 9765 9780... 

The sensing methods summarized thus far are intended for absorption detection of molecules in the ambient, but molecules (or indeed thin films) on the microresonator surface can also be detected. In particular, if the surface is covered to such an extent that the optical energy absorbed heats the microresonator, the resulting thermal bistability in the frequency-scan response can be used to determine the absorption and/or thickness of the thin-film coating. This and surface characterization by measurement of the thermal accommodation coefficient were described in Sect. 5.5. These methods offer quite precise measurement, provided that certain reasonable and easily implemented assumptions are satisfied. 

The FPI principle can also be used to develop thin-film-coating-based chemical sensors. For example, a thin layer of zeolite film has been coated to a cleaved endface of a single-mode fiber to form a low-finesse FPI sensor for chemical detection. Zeolite presents a group of crystalline aluminosilicate materials with uniform subnanometer or nanometer scale pores. Traditionally, porous zeolite materials have been used as adsorbents, catalysts, and molecular sieves for molecular or ionic separation, electrode modification, and selectivity enhancement for chemical sensors. Recently, it has been revealed that zeolites possess a unique combination of chemical and optical properties. When properly integrated with a photonic device, these unique properties may be fully utilized to develop miniaturized optical chemical sensors with high sensitivity and potentially high selectivity for various in situ monitoring applications. 

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