Dispersion of the nonlinear second-order optical susceptibility of organic systems

Abstract

Dispersion experiments of the second-order nonlinear optical susceptibility are reported for two fundamentally important organic systems $p$-nitroaniline and 2-methyl-4-nitroaniline for which detailed quantum-mechanical calculations have been completed. The frequency-dependent measurements were performed by dc-induced second-harmonic generation in liquid solutions with the use of a tunable (0.2-2 \ensuremath{\mu}m) laser source by stimulated Raman scattering from hydrogen gas. Accounting for dipole-mediated interactions between the molecular sites and the surrounding solvent medium provides quite satisfactory agreement between the experimental and theoretical results, demonstrating that the origin of exceptional nonlinear optical responses of organic systems resides in special highly charge-correlated electron states.