High-Reflection Structures

A reflective surface can be specular (mirror-like, glossy) or diffuse. Fig. 2.34. The retroreflective surfaces return the beam in the direction the incident light came from. Regardless of the type of reflectivity, most of the radiation is returned from the illuminated surface toward the radiation source. This radiation can be practically unchanged or can be shifted in liequency spectrum, either toward higher or toward lower frequencies ( hot and cold mirrors). 

Another issue of interest is the angular dependence of reflection coeflftcient. For some structures, this dependence is very strong and large values of / are obtained only for near-normal incidence, thus furnishing surfaces with very high directivity of reflected light. The other pole are the surfaces offering near-omnidirectional or even fully omnidirectional reflectivity. 

A surface can have significant diffuse reflectance, dependent on the scattering properties of the material. There is a range of possible intermediate situations between a fuUy specular and fully diffused optical reflector surface. 

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The simplest and the most obvious approach is to utilize some kind of high-reflection structure to change the path of the beam and return it back to the active... 

The strained hydrocarbon [1,1,1] propellane is of special interest because of the thermodynamic and kinetic ease of addition of free radicals (R ) to it. The resulting R-substituted [ 1.1.1]pent-1-yl radicals (Eq. 3, Scheme 26) have attracted attention because of their highly pyramidal structure and consequent potentially increased reactivity. R-substituted [1.1.1]pent-1-yl radicals have a propensity to bond to three-coordinate phosphorus that is greater than that of a primary alkyl radical and similar to that of phenyl radicals. They can add irreversibly to phosphines or alkylphosphinites to afford new alkylphosphonites or alkylphosphonates via radical chain processes (Scheme 26) [63]. The high propensity of a R-substituted [1.1.1] pent-1-yl radical to react with three-coordinate phosphorus molecules reflects its highly pyramidal structure, which is accompanied by the increased s-character of its SOMO orbital and the strength of the P-C bond in the intermediate phosphoranyl radical. 

A breakthrough came in 1988 when Hamada et al. demonstrated that a dye layer with a relatively low optical absorbance and high reflectance at the recording wavelength could be interposed between the substrate and reflecting layer of a CD-ROM type structure, and, with appropriate optimization of the optical properties, this would record and reproduce in accordance... 

A drawback of the MD-class BioCD is the microfabrication required to pattern gold spokes on the disc to set the quadrature condition. To remove the microfabrication, an alternative means to establish a quadrature condition uses adaptive optical mixers in the far field to establish and lock phase quadrature. In this case, the disc can be a high-reflectance antinode surface with protein patterned directly on the disc surface without any need for surface structuring. As the disc spins, the immobilized protein causes phase modulation that is detected in the quadrature condition set up by the adaptive mixer. 

Reflection contrast Reflection-imaging microscopy Field ion microscopy Quantification in gap between light and em microscopies Useful for imaging highly reflective particles such as silver grains in autoradiographs Atomic structure of crystals Immunoelectron Localization of cellular antigens... 

The simplest recording medium is a bilayer structure. It is constructed by first evaporating a highly reflective aluminum layer onto a suitable disk substrate. Next, a thin film (15-50 nm thick) of a metal, such as tellurium, is vacuum deposited on top of the aluminum layer. The laser power required to form the mark is dependent on the thermal characteristics of the metal film. Tellurium, for example, has a low thermal diffusivity and a melting point of 452 °C which make it an attractive recording material. The thermal diffusivity of the substrate material should also be as low as possible, since a significant fraction of the heat generated in the metal layer can be conducted to the substrate. For this reason, low cost polymer substrates such as poly (methylmethacrylate) or poly (vinyl chloride) are ideal. 

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