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OPTICS OF J-AGGREGATES: AN ENDLESS PLAYGROUND FOR COLLECTIVE EXCITATIONS

Molecular J-aggregates are self-assembled systems of tens to thousands of molecules kept together by electrostatic forces. The strong intermolecular interactions within aggregates, lead to collective excitations that are spread over many molecules. These collective excited states give rise to interesting collective optical and nonlinear optical properties and reveal intriguing temperature dependent dynamics, not observed for non-aggregated molecules. Examples are motional narrowing, exciton superradiance, and optical bistability. All of these phenomena have been the topic of many experimental and theoretical studies over the past twenty years. In this colloquium, I will introduce these systems, explain why they are interesting, and show that relatively simple phenomenological models for the relevant quantum mechanical degrees of freedom give a surprisingly good description of a wide variety of optical experiments. The key players in this talk are Frenkel excitons, vibrations, photons, disorder, and two-quantum excitations.