The third-order nonlinear optical (NLO) response of colloidal “quantum-dot” cadmium sulfide (CdS) nanocrystals has been measured both in solution and in Langmuir−Blodgett (L−B) multilayers of a polydiacetylene (PDA) “alloy” NLO polymer. The PDA “alloy” was composed of a tailored mixture of diacetylene monomers having carboxylic acid “head” groups and benzamide “head” groups, in each case separated from the diacetylene moiety by an eight-carbon spacer group, so as to disrupt the strong aggregation, leading to excessive light scattering, that normally occurs with L−B films of carboxyl-terminated PDAs. It is shown that at 530 nm the nonlinear refractive index (n 2) of a nanocomposite PDA film containing thiophenol-capped CdS nanocrystals, as measured by both Z-scan and degenerate four-wave mixing (DFWM) techniques, is 11 × 10-8 cm2/MW, while that of an undoped PDA film is only 3 × 10-8 cm2/MW. This demonstrates the concept of a “nonlinear−nonlinear” optical material, in which the NLO response of an embedded quantum-dot semiconductor can add constructively to that of a NLO polymer having complementary wavelength characteristics.

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The polydiacetylenes have received much attention in recent years because of their large non -resonant third -order nonlinear optical response and because of the flexibility they offer in designing tailored molecular structures for specific applications. However, the fabrication of these materials into devicequality thin films has proven to be difficult, largely because of their propensity for forming highly scattering polycrystalline solids (with the exception of the few soluble polydiacetylenes, such as 3-or 4 -BCMU, which can be spin-coated by conventional techniques, albeit in the less-active yellow form of the polymer).We have addressed this problem by developing a family of polydiacetylene "alloys ": polymers derived from mixtures of similar diacetylene monomers having identical positioning of the diacetylene unit within the hydrocarbon chain, but with different "head" groups that provide varying degrees of affinity to water. The rationale here has been to design mixtures that would form a uniform, quasiamorphous monolayer on the water surface in Langmuir-Blodgett multilayer fabrication, and thus avoid the scattering losses associated with grain boundaries that form within the monolayer on the water surface. To date, multilayer films up to 0.4 um thick have been fabricated by this approach, and photolithographic techniques have been developed for patterning strips down to 4 um wide in the films, without converting the blue form of the polydiacetylene to the red form in the process. Moreover, high quality films of polydiacetylene have been deposited successfully onto curved surfaces, such as polycarbonate lens blanks, using these compositions.

Colloidal, "quantum-dot" semiconductormaterials, as exemplified by the sharp-cut yellow-red filter glasses which contain small (less than 100 A diameter) crystallites of cadmium sulfide and/or cadmium selenide, are of current interest because of the large near-resonant third-order nonlinear optical response they exhibit. Because these semiconductors are generally soluble only in inorganic media, such as silicate glasses, they have not been considered widely as candidates for inclusion in organic polymer waveguides. We report here the initial results of a continuing study in which very small colloidal particles of both cadmium sulfide and cadmium selenide (with crystallite diameters of typically 30-50 A) have been prepared by the reverse-micellar technique and then "capped" with a variety of organic reagents, using the methodology described several years ago by Brus and Steigerwa1d14. By varying the organic capping reagent, we find that we are able to produce significant variations in both the solublilty and the nonlinear optical response of the resulting composite materials. In particular, we find that the solubility of the particles in organic polymers such as PMMA and polycarbonate varies greatly with the nature of the capping reagent, with corresponding changes in the nonlinear refractive index of the composites. The feasibility of developing organic nonlinear optical waveguides based on this approach will be discussed.

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