Dispersion of second-order optical nonlinearity in chromophoric self-assembled films by optical parametric amplification: Experiment and theory

The dispersion of the second-order optical nonlinearity in chromophoric self-assembled stilbazolium films has been characterized in detail as a function of fundamental wavelength from 800 to 1600 nm using an optical parametric amplifier-based measurement system. The second-harmonic generation (SHG) spectrum exhibits a distinctive two-photon resonance at ω=1.3 eV (960 nm). The maximum in the second-order susceptibility coincides with a low-energy chromophore-centered charge-transfer 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 the χ⁽²⁾ response is dominated by an excitation along the long axis of the stilbazolium chromophore from the highest occupied to the lowest unoccupied molecular orbital.