Dipolar resonances in small-particle fractal aggregates may be localized in sub-wavelength spatial regions, leading to enhanced local fields. Local field enhancements, in turn, result in the enhancement of both linear and nonlinear optical processes. Doping the aggregates into microcavities (e.g., liquid microdroplets) further increases the local fields because of trapping by mirocavity resonant modes. Nanoparticle-microcavity composites are expected to possess unique characteristics including extremely large linear and nonlinear optical susceptibilities and strong dependence of the fractal eigenmodes on the frequency and polarization of the exciting laser and on the structure of the microcavity resonances. Investigations of nanocomposite materials will provide insight into the effect of morphology on spectral properties. The composites themselves have potential applications to photonics, information technology, signal processing, aerosol science, and remote sensing. This project will carry out experimental and theoretical studies of a variety of optical effects including linear and nonlinear Raman scattering, fluorescence and lasing, and other third-order processes including degenerate four-wave mixing; other, more fundamental, studies will include Anderson localization and cavity quantum electrodynamics. %%% Aggregates of small particles are widely found in nature and may also be fabricated in the laboratory. In the process of formation many of these aggregates form frothy structures that possess so called "scale invariance", that is, they have the same appearance when viewed over a wide range of sizes. Such aggregates are examples of "fractals" and include commonly occurring, materials such as smokes, foams, polymers, and DNA. A second constituent of the composite is exempli fied by the simple raindrop (that is, a small, spherical liquid droplet). The composite material itself, consisting of liquid droplets containing fractal aggregate inclusions, may form naturally or may be the result of a specific laboratory procedure. A number of recent experimental and theoretical studies suggests that these composite materials possess unusual optical properties, resulting from the fact that each constituent (i.e., the fractal aggregate and the microdroplet) possess so-called electromagnetic resonances. Electromagnetic resonances are responsible for many unusual optical phenomena and generally lead to an amplification of the light emitted by the resonant system. General theoretical arguments suggest that the composite materials should exhibit enhanced resonance behavior, they have many potential applications to such diverse fields as information technology, signal processing, and atmospheric monitoring. In this project, the PIs will carry out both experimental and theoretical investigations of these novel composite materials. ***
