The optical properties of nanoscale composite materials are often quite different from the properties of the constituent materials from which the composite is constructed. The formation of composite materials thus constitutes a means for engineering new materials with desired optical properties. In this paper we review theories and models that have been devised for relating the linear and nonlinear optical properties of composite materials to those of the constituent materials and to the morphology of the composite structure, and we review experimental studies aimed at validating these models. Morphologies that are explicitly discussed include those of Maxwell Garnett and of Bruggeman, layered structures, and fractal structures.

%'e present experimental results which demonstrate that the effective third-order susceptibility of a composite optical material can exceed those of the materials from which it is constructed. In particular, we have formed a composite of alternating, sub-wavelength-thick layers of titanium dioxide and the conjugated polymer poly(p-phenylene-benzobisthiazole), and find that its nonlinear susceptibility exceeds that of its more nonlinear constituent by 35%. The enhancement of the nonlinear susceptibility, which under more ideal but still realistic conditions can be as large as a factor of lo, can be understood as a consequence of local field corrections.There is a great need for nonlinear optical materials with large nonlinear coefficients and fast response. Many applications of nonlinear optics that have been demonstrated under controlled laboratory conditions could become practical for technological uses if such materials were available. Nonlinear optical switching devices for use in photonics and real-time coherent optical signal processors are examples of applications of nonlinear optics that would benefit from the development of fast, low-loss materials with large values of the third-order nonlinear susceptibility Xt3l [1]. The most common approach to the development of new nonlinear optical materials involves the search for materials in which the constituent molecules possess an inherently large nonlinear response [2]. In contrast, in this Letter we describe a technique that can be used to increase the g(3~v alue of a nonlinear optical material by forming a composite of that material and another material having a different value of the linear refractive index.The two constituent materials can be optically lossless, and the response time of the composite is essentially the same as that of the nonlinear constituent. The two materials are intermixed on a distance scale of the order of 50 nm, which is much larger than an atomic dimension but much smaller than the wavelength of light used in our experiment. Consequently, the structural properties of each constituent material are essentially the same as those of a bulk sample of that material, but the propagation of light through the composite can be described by effective values of the linear and nonlinear optical susceptibilities that are obtained by performing suitable volume averages. The reason why this technique leads to an enhancement of~(~is that the electric field amplitude of an incident laser beam becomes nonuniformly distributed between the two constituents of the composite, and under suitable conditions the electric field strength within the more (3) jeff eff 2 2 err( 3) f-x. a nonlinear constituent will exceed the spatially averaged field strength. Under these conditions, the effective thirdorder susceptibility g, «of the composite can exceed that (3) of either of its constituents.We have previously performed detailed theoretical studies of composite materials of the sort mentioned above both for the case in which small inclusion particles are embedde...

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The Montreal Urban Community Wastewater Treatment Plant (MUCWTP) located in Montreal, Quebec, Canada, uses physicochemical treatment processes prior to discharging wastewater into the St. Lawrence River via an outfall tunnel of 2 hours retention time. Although chlorination facilities exist, they are not being used, and the MUCWTP is seeking alternative methods for disinfection to achieve a 2-to 3-log fecal coliform reduction. Liquid chemical disinfectants were attractive options because of their low capital costs. This led to an investigation of the feasibility of using hydrogen peroxide or peracetic acid.A method for measuring peroxycompounds (hydrogen peroxide or peracetic acid plus hydrogen peroxide) was developed using the peroxidase-based oxidation of 2,2Ј-azino-bis(3-ethylbenz-thiazoline-6sulfuric acid) diammonium salt (ABTS) with hydrogen peroxide. The validity of the method was confirmed using effluent from the MUCWTP. Recovery was higher than 90% for peracetic acid levels as low as 1.0 mg/L. Quenching of hydrogen peroxide was achieved with 50-mg/L catalase; quenching of peracetic acid was achieved with 100 mg/L of sodium thiosulfate, followed by 50 mg/L of catalase.Batch disinfection tests were conducted on MUCWTP effluent. Hydrogen peroxide and peracetic acid in wastewater over time could be modeled as a second-order decay, with the decay "constant" being a function of the initial concentration of peroxycompounds. This function was the same for both hydrogen peroxide and peracetic acid, possibly indicating similar decomposition pathways in wastewater matrices.Disinfection was modeled using a modified Hom equation. Required doses of hydrogen peroxide to reach the target fecal coliform levels ranged from 106 to 285 mg/L, with the higher doses occurring when ferric chloride instead of alum was used as the coagulant. Hence, hydrogen peroxide was infeasible as a disinfectant for this application. On the other hand, the peracetic acid dose needed to achieve the target fecal coliform level was only 0.6 to 1.6 mg/L. Therefore, peracetic acid seems to be a promising disinfectant for physicochemical or primary effluent, or combined sewer overflows. Water Environ. Res., 74, 33 (2002).

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