Thin-film interference filters

In the mid-IR, unlike NIR, it is customary to use microns or micrometers (um) for the wavelength scale, not nanometers (nm). Also, for filter-based instruments the wavelength scale is traditionally used instead of the wavenumber (cm ) frequency based scale this tends to be historical based on thin film interference filter technology. 

Light separation by filters can be performed with coloured glass or dyed gelatine filters. The quality of separation and the thermal stability of the filters, however, have been improved considerably by the use of thin film interference systems, see [Id, 2, 6, 115]. 

Furthermore, thin-film interference filters often suffer from a polarization sensitivity, which can produce unwanted selectivity in applications where polarization sensitivity is desired (see Section 3.4). Fortunately, a number of suppliers (Chroma Technology Corp., VT, USA for example) are now becoming familiar with the requirements of filters for single molecule applications and are constantly developing the technology. Compared to other types of filters the interference type cannot be beaten on cost good quality filters suitable for single molecule work are available for around US 150. 

H. Angus Macleod, 2001, Thin-Film Optical Filters, Third Edition, ISBN 0 7503 0688 2, loP, Institute of Physics Publishing, Bristol and Philadelphia Alfred Thelen, 1988, Design of Optical Interference Coatings, ISBN 0-07-063786-5, McGraw-Hill Book Compagny... 

Chloride, sulfate, and other anions may affect the removal of arsenic by nanofiltration. The effects often depend on the composition of the nanofilter. Specifically, increasing NaCl concentrations actually improves arsenic removals with polyamide thin-film composite filters. On the other hand, NaCl solutions may interfere with the removal of arsenic with sulfonated polysulfone thin-film composite nanofilters (Shih,... 

The FM of the MSOF fiber excites several higher-order modes in the tapered region, while the second tapered region acts as a spatial filter for the intermodal interference. An 8 nm Pd thin film deposited in a vacuum chamber on the LPG formed the sensor. Its responses to different concentrations of H2 in nitrogen are shown in Fig. 17a, b, which summarizes the dependence of the intensity changes on the H2 concentration. The response time of the sensor to reach a 0.9 level of transmission change was found to be about 10 s. 

Interference filters consist of two layers of glass on whose inner surfaces a thin semitransparent film of metal is deposited and an inner layer of a transparent material such as quartz or calcium fluoride. Radiation striking the filter exhibits destructive interference, except for a narrow band of radiation for which the filter is designed to transmit. The bandwidth of the filters decreases as the transmitted radiation increases. 

The main commercial use of solid SiO is as a vapor-deposition material for the production of SiOx thin films for optical or electronic applications (antireflective coatings, interference filters, beam-splitters, decorative coatings, dielectric layers, isolation layers, electrodes, thin-film capacitors, thin-film transistors, etc.), for diffusion barrier layers on polymer foils or for surface protection layers.Other uses for SiO have been proposed, such as the substitution of elemental silicon in the Muller-Rochow process for the production of organosilicon halides, because solid SiO can be produced at lower temperatures than elemental silicon. 

Based on the same concept [449], sensing response, response time, and recovery time for selective ethylene gas detection have been improved by b-oriented SIL-1 layer prepared by hydrothermal synthesis onto SnOj thin film (100 nm) sensors. By preferential adsorption, the SIL-1 layer selectively filters a nonpolar analyte (ethylene) from the polar water leading to an improved selectivity, and hence a sensor with less moisture interference can be obtained. The SIL-1 filtering layer also acts as a preconcentrating media that readily increases the ethylene detection sensitivity exhibiting response and recovery times of 14 and 144 s, respectively. 

In Subsection 5.6.a we review the theory of multilayer thin films. In Subsection 5.6.b this theory is applied to the design of antireflection coatings for infrared windows, lenses, and other components. The same theory is used again in Section 5.6.C to find suitable beam dividers of the free-standing, self-supporting type as well as of the type requiring a transparent substrate. Subsection 5.6.d deals with interference filters and Fabry-Perot interferometers. 

The thin film theory developed in Subsection 5.6.a is directly applicable to the design of interference filters and Fabry-Perot interferometers. These devices are similar in concept and, therefore, are treated together. The normalized reflected amplitude for a three layer film of refractive indices 2, and no [see Eq. (5.6.21)] is applied first... 

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