Origin of the highly efficient third-order optical response in small organic molecules

The search for materials with high optical nonlinearity has driven decades of research, with little success in developing materials that are suitable for third-order nonlinear optical applications that utilize the intensity-dependent refractive index or intensity-dependent absorption, for example. Organic materials, with their highly delocalized electron systems that lead to large nonlinearities and fast (10-15 s) optical response times and the relative ease at which their molecular properties can be tailored (allowing certain molecular properties to be isolated and optimized through molecular structure variations), are ideal candidates for the next generation of nonlinear optical materials. These materials have led to the largest molecular nonlinearities measured, but still remained several orders of magnitude below their theoretical limits. Our aim in this work is to approach the absolute limit, but also to keep the molecular size small. This research direction should lead to a dense packing of molecules with high nonlinearity in the solid state, which will then lead to the large bulk nonlinearities necessary for all-optical applications. With donor-substituted cyanoethynylethene molecules, we have measured off-resonant nonlinearities within a factor of 50 of their theoretical limits and predict that solid-state devices made from these materials can be up to an order of magnitude larger than other materials made from organic molecules. By measuring a large number of molecules with different structural variations, we have isolated the electron donor and acceptor structures as necessities for large third-order nonlinearities in small molecules. Our best molecule has a molecular third-order polarizability per mass of g&d5; rot = 6.5 x 10-23 m5V-2Kg-1, the highest value measured to date. We have also measured our molecules on resonance, which leads to measurements of the two-photon absorption cross-section sigma TPA. These measurements give a value of 9 x 10 -48 cm4s/photon for our best molecule, which is an order of magnitude larger than a chromophore optimized for two-photon absorption. Because of the record high results measured in these molecules, along with their ability to be vapor deposited into integrated devices, we suggest that this family of molecules may be the predecessors to the next generation of applied third-order nonlinear optical materials.