Quarter-wave coatings

Figure 14. Reflectance of a 100 nm thick silver coating enhanced with two quarter-wave Sio2 layers.

The structures that have evolved for ablative-mode optical discs make use of interference effects to minimize the reflectance (R) of the disc in the absence of a hole. A typical ablative-mode optical disc has the structure shown in Figure 5.51. The substrate is an optically transparent material such as polycarbonate, poly(methyl methacrylate), poly(ethylene terephthalate), or poly(vinyl chloride), topped by a subbing layer to provide an optically smooth (to within a fi-action of a nanometer) surface for the recording layer. A metal reflector (typically aluminum) is then incorporated next to a transparent dielectric medium such as spin-coated poly(a-methyl styrene) or plasma-polymerized fluoropolymers. This dielectric spacing layer serves both to satisfy the quarter-wave (2/4) antireflection conditions and to insulate thermally the A1 reflector from the top absorbing layer where the information pits are created. 

There are several studies on the multiplex interference layer. Shen et al. prepared multilayer highly reflective dielectric coatings consisting of alternating layers of quarter-wave... 

Interference mirrors are dielectric thin film coatings where low- and high-refractive index layers alternate. The optical thickness of each of the layers is equal to quarter-wavelength QJAn). They are denoted as distributed Bragg mirrors or distributed Bragg reflectors (DBR), sometimes simply as Bragg mirrors. Other names include quarter-wave mirrors (QWM), quarterwave stacks (QWS) and highly reflective (HR) layers. 

Imperfections or nonidealities of the optical components in an ellipsometer can cause errors in A and il/. Quantitative analyses of errors have been made by Jerrard for inexactness of the quarter-wave plate, by Azzam and Bashara" for nonideal optical activity of the quarter-wave plate and surface roughness, and by Smith" for finite bandwidths of the monochromatic light source as well as the source polarization. Archer and Shank" and Yolken, Waxier, and Kruger considered the effect of multiple reflection within the compensator plate. It is reported that antireflection coatings on the surfaces of the compensator plate are beneficial. Aspnes" treated in a formal way the first-order effects from... 

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-... 

Single-layer and multilayer coatings of oxides are also used to enhance reflectivity in the near IR. These coatings work because of optical interference effects and usually have a thickness about one-quarter of the wavelength of the radiation such that the primary wave reflected off the first interface is 180° out of phase with the secondary wave reflected from the second interface. The result is destructive interference of the two waves. Thin films of Ti02,... 

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