DISPERSION OF 2ND-ORDER OPTICAL NONLINEARITY IN CHROMOPHORIC SELF-ASSEMBLED FILMS BY OPTICAL PARAMETRIC AMPLIFICATION - EXPERIMENT AND THEORY

The dispersion of the second-order optical nonlinearity in chromophori c self-assembled stilbazolium films has been characterized in detail a s a function of fundamental wavelength from 800 to 1600 nm using an op tical parametric amplifier-based measurement system. The second-harmon ic generation (SHG) spectrum exhibits a distinctive two-photon resonan ce at homegaBAR = 1.3 eV (960 nm). The maximum in the second-order sus ceptibility coincides with a low-energy chromophore-centered charge-tr ansfer one-photon excitation at 480 nm (2.6 eV). The experimental SHG dispersion values compare favorably with theoretical results computed using a semiempirical sum-over-states formalism. These indicate that t he chi(2) response is dominated by an excitation along the long axis o f the stilbazolium chromophore from the highest occupied to the lowest unoccupied molecular orbital.