Reflectivity from multilayer film

Antireflective (AR) coatings are required on optics to reduce the reflective surface losses. Vitreous siHca coatings in the form of porous or multilayer films are used extensively in this appHcation. Antireflective coatings have been developed which employ coUoidal fused siHca sol—gel particles made from organometaUic materials (253). 

The inner structure of polyelectrolyte multilayer films has been studied by neutron and X-ray reflectivity experiments by intercalating deuterated PSS into a nondeut-erated PSS/PAH assembly [94, 99]. An important lesson from these experiments is that polyelectrolytes in PEMs do not present well-defined layers but are rather interpenetrated or fussy systems. As a consequence, polyelectrolyte chains deposited in an adsorption step are intertwined with those deposited in the three or four previous adsorption cycles. When polyelectrolyte mobility is increased by immersion in NaCl 0.8 M, the interpenetration increases with time as the system evolves towards a fully mixed state in order to maximize its entropy ]100]. From the point of view of redox PEMs, polyelectrolyte interpenetration is advantageous in the sense that two layers of a redox polyelectrolyte can be in electrochemical contact even if they are separated by one or more layers of an electroinactive poly ion. For example, electrical connectivity between a layer of a redox polymer and the electrode is maintained even when separated by up to 2.5 insulating bUayers [67, 101-103]. 

Devices that use uricase in a dry reagent format to measure uric acid have also been described. For example, a multilayer film system employs uricase and peroxidase separated by a semipermeable membrane from a leuco dye that is oxidized to form a colored product. A ceUulose matrix pad system employs uricase, peroxidase, and MBTH as oxygen acceptor the system employs a diluted plasma sample, which helps to reduce interferences, although ascorbic acid was shown to be a significant interferant. A third system incorporates separation of plasma from red cells and uricase, peroxidase, and a substituted phenol to measure uric acid. AH three systems employ a reflectance meter system to facilitate accurate and precise quantitation of the color change. 

Figure 14.4 Schematic cross-sectional view of a multilayer film reflector. 0 is the angle of incidence of the light relative to the surface. Reflection from the first few interfaces between the more reflective material (dark gray) and the more transparent material (light gray) are shown. (Reprinted with permission from Ref. 40. 2009 McGraw-Hill.)...

Ellipsometry is an optical technique that detects the change of the polarization state when light is reflected from a surface. For rather simple systems like transparent films on reflecting substrates, film thickness and refractive index can be determined with high accuracy. More complicated samples (e.g., multilayer structures or layers with a graded index of refraction on a reflective carrier) can be characterized with a sufficient set of independent experimental data obtained for multiple angles of incidence and/or multiple wavelengths (spectroscopic ellipsometry). With a liquid cell, ellipsometry can be performed also in aqueous environments. 

Another application of microspectroscopy is characterization of laminated polymer films. Multilayer polymer films of different fimctionality are common in the packaging industry and identification of the different layers is of commercial importance. The layers vary from about 1 /xm in thickness (adhesive) to 10-50 /zm and there are often several layers. If there are only two layers, attenuated total reflectance (ATR) methods can be used for identification. For films with more than two layers, the normal procedure is to microtome a 10-20 fj,m cross section for edgewise analysis using microspectroscopy. A comparison of infrared and Raman microspectroscopies for some multilayer films has been pubhshed (Fig. 25) (193). 

The propagation matrix F describes the modification of the wave field A upon propagation from coordinate z to coordinate z + dz. It is a multidimensional matrix with dimension given by the number of the open scattering channels. This formalism is successfully applied not only to describe the propagation of light in forward direction but also in case of X-ray diffraction from single crystals, and reflection from surfaces, thin films, and multilayers. 

The NTS monolayer which oriented the reactive vinyl groups at the monolayer surface was prepared by the Langmuir method. The NTS monolayer is in a hexagonal crystalline state at 293 K, and the size of the NTS mono-crystalline domain extended to a distance of 10 pm. The stable hydrophilic NTScoqh monolayer was obtained by oxidation of the NTS monolayer. The NTS multilayer film was constructed by repetitions of the processes of oxidation of the vinyl end group and depositions of the NTS monolayers. It was shown from X-ray reflectivity and B-FT-IR measurements that the NTS multilayer film has a well-ordered layer structure. 

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,... 

In the context of this study both, SHG [49] and enhanced Raman scattering [50], from multilayer samples have been studied. These substrates, which consist of a silver island film separated from a silver mirror by a dielectric spacer layer, excel by remarkable reflectivity and absorption properties that have been accoimted for by appropriate modelling [51]. 

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