Optical nonlinear filters

Figure 10.5 Fast sol-gel monoliths as optical nonlinear filters of CNT-doped FSG (red curve) compared with pure FSG (black curve), (a) Transmission spectra. (b) Optical transmission response as a function of pump intensity, (c) Nonlinear response in z-scan measurement for CNT-doped sol-gel only.

The cadmium chalcogenide semiconductors (qv) have found numerous appHcations ranging from rectifiers to photoconductive detectors in smoke alarms. Many Cd compounds, eg, sulfide, tungstate, selenide, teUuride, and oxide, are used as phosphors in luminescent screens and scintiUation counters. Glass colored with cadmium sulfoselenides is used as a color filter in spectroscopy and has recently attracted attention as a third-order, nonlinear optical switching material (see Nonlinear optical materials). DiaLkylcadmium compounds are polymerization catalysts for production of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVA), and poly(methyl methacrylate) (PMMA). Mixed with TiCl, they catalyze the polymerization of ethylene and propylene. 

By the sol-gel-process, inorganic glassy and hybrid polymeric materials are accessible at comparatively low temperatures [1], Therefore, organic molecules or dyes can easily be incorporated into the oxide matrix. This combination is especially attractive for the development of the following devices optical filters, solid-state lasers, optical switches, nonlinear optical laser hosts, optical data storage media, and photoconductive devices and films [2]. 

In one application (Maitre, 1981), the parallelism of noncoherent optical convolution methods is combined with video techniques in a totally analog application to images. Matsuoka et al (1982) have developed a hybrid optical-video-digital system based on similar principles. Pipeline processors that perform image convolution at high speed are ideal for digital applications. The inherent simplicity of the method adapts it to high-speed implementations by use of other hybrid techniques. It is even possible to devise a transversal filter that has all the desirable nonlinear properties of the method. We describe this filter in the next section. 

Y. Mitsuhashi, Saturable Fabry-Perot filter for nonlinear optical image processing, Opt. Lett., 6, 111-113 (1981). 

Photochromic materials based on different classes of spiropyrans (SPs) are widely used in various fields of science and technology, such as in the production of light filters regulating luminous fluxes, as photochromic organic media for processing optical information, for photochromic optics, and in the production of nonlinear optical materials. In recent years, the study of new SPs has been conducted mainly in two directions, namely, the search for new classes of SPs and structural modification of the known systems to improve their basic characteristics (quantum yield of photoconversion, the stability of the photoisomer produced, the number of cycles of operations). Only a profound comprehension of mechanisms of photoinitiated rupture of the Cspiro—O bond, structural isomerization, ways of stabilizing the photoisomer, routes of its breakdown, and influence of the structure of SPs on their properties can provide the basis for purposeful research in this area. Despite the vast number of investigations in this area, the mechanisms of the photochromic conversion of SPs and the influence of structural features on their photochemical properties are not well understood. This complicates the search for and synthesis of new SP classes. 

Figure I. Experimental arrangement for the observation of SFG from a chiral liquid. The optical elements shown include a beam splitter (BS), a filter (F), a photomultiplier tube (PMT), and a nonlinear crystal (BBO)...

The emission intensity of fluorescence depends on the product of the molar extinction coefficient, the optical path length, the solute concentration, the fluorescence quantum yield of the dye, and the excitation source intensity. In dilute solutions, the intensity is linearly proportional to these parameters. When sample absorbance exceeds about 0.05 in a 1-cm path length, the relationship becomes nonlinear and measurements may be distorted by artifacts such as self-absorption and the inner-filter effect. 

Liquid crystalline polyesters with 3-methylphthalimide and chromophore side chains, having good orientation stability and mechanical properties, were claimed for making nonlinear optical devices like filters, polarisers, waveguides and others [274]. 

The key featme of Daly s visible-difference predictor is a model of the human visual system that concentrates on the lower-order processing of the system such as optics, retina, lateral geniculate nucleus and some higher-order processing in striate cortex. The model contains three main components (i) an amplitude nonlinearity (in image-processing terms, a point process), (ii) the contrast sensitivity functions (implemented as a filter process), and (iii) a detection process which models masking processes within the visual system. 

The nonlinear optical properties of semiconductor particles was first studied with commercial color filters, which contain nominally CdSxSei-x particles of 100-1000 A diameter [88]. The reported large nonlinearity attracted much interest and soon the study was extended to CdS clusters in the quantum-confined size regime (i.e., below 60 A for CdS) [89,90]. 

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