Molecular Design for Third-Order Nonlinear Optics

We discuss recent developments in the design of molecules for applications in third-order nonlinear optics with emphasis on all-optical signal processing and two-photon absorption. We especially concentrate on functional substitution patterns and conjugation length expansion. We show that low molecular symmetry with regard to the conjugation path of the delocalized electrons was found to be a good guideline towards linearly conjugated molecules with large second-order hyperpolarizabilities γ. We show that this guideline is also valid for two-dimensionally conjugated systems and that the observed effects can be explained by the symmetry of the electronic wavefunctions. We present scaling laws and critical conjugation lengths of rod-like molecules with electrons delocalized over a one-dimensional path and show that the exponent tends to be constant for various polymers in the transparency range and that the values presented here are of similar magnitude for various organic materials systems. Finally, we discuss different materials systems with regard to the figures of merit relevant for all-optical signal processing.