Design of Polymethine Dyes with Large Third-Order Optical Nonlinearities and Loss Figures of Merit

Dying by Design To make optical-switching applications a reality, losses from scattering and other absorption processes have to be minimized. Hales et al. (p. 1485, published online 18 February; see the Perspective by Haque and Nelson) present a strategy to explore the refraction and absorption properties of a group of cyanine dyes for designing materials that have properties corresponding to technologically interesting telecommunications windows. The optical properties of the cyanine molecule was controlled by adding heavy chalcogen atoms (selenium) into the end groups of the molecular structure. While producing a series of molecules meeting criteria for feasible application, the work also demonstrates a route to improve the performance of nonlinear optical materials. Nonlinear optical materials are designed and characterized for potential applications in all-optical switching. All-optical switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. We present a design approach that involves enhancing the real part of the third-order polarizability (γ) of cyanine-like molecules through incorporation of polarizable chalcogen atoms into terminal groups, while controlling the molecular length to obtain favorable one- and two-photon absorption resonances that lead to suitably low optical loss and appreciable dispersion enhancement of the real part of γ. We implemented this strategy in a soluble bis(selenopyrylium) heptamethine dye that exhibits a real part of γ that is exceptionally large throughout the wavelength range used for telecommunications, and an imaginary part of γ, a measure of nonlinear loss, that is smaller by two orders of magnitude. This combination is critical in enabling low-power, high-contrast optical switching.