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Reduced dimensionality and quantum confinement in conjugated organic and polymer structures enhance the effects of electron correlation on virtual electronic excitation processes and nonlinear-optical responses. A microscopic many-electron description of the third-order susceptibilities Yijkl(-W4; w1, 2, c3) of conjugated structures is reviewed for one-dimensional chains and extended to two-dimensional conjugated cyclic structures. Electron correlation effects in effectively reduced dimensions result in highly correlated 7-electron virtual excitations that lead to large, ultrafast nonresonant nonlinear-optical responses. The increase of dimensionality from linear to cyclic chains is found to reduce the nonresonant isotropic third-order susceptibility yg. Resonant experimental studies of saturable absorption and optical bistability in ultrathin films of quasi-two-dimensional naphthalocyanine oligomers are also presented. In the saturable-absorption studies, the resonant nonlinear refractive index n was measured to be 1 X 10-4 cm2/kW in the wavelength range of operating laser diodes. Based on this result, electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry-Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear-optical medium.

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Using a recently developed many-electron theory of second and third order nonlinear optical susceptibilities, microscopic descriptions of γxxxx(-3ω;ω,ω,ω) are presented for two conformations of polyenes. The importance of electron correlations and the effects of variation in conformation and chain length are discussed.

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