All-optical signal processing / switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. At the molecular level, it means maximizing the the real part of the third-order polarizability (γ) and minimizing its imaginary part. Practical applications require the figure of merit, Re(γ)/Im(γ), to be larger than 4p. Cyanine dyes such as the bis(seleno-pyrylium) heptamethine dye can exhibit 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 can be two orders-of-magnitude smaller ; this combination is critical to enable low-power, high-contrast optical switching. In this presentation, we will first provide a quantum-chemical description of the real and imaginary nonlinear optical properties of relevant polymethine-type molecules. We will then discuss the strategies that need to be followed in order to translate the properties of the isolated molecules into the solid state. We will describe some recent examples based on zwitterionic cyanines  and Pd(PPh3)2Cl-substituted cyanines.
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