Second-order non-linear optical properties of new organic conjugated molecules

Abstract

Both real and imaginary parts of the second-order non-linear optical coefficients for a wide range of new organic conjugated materials have been measured. The technique of electric-field-induced second-harmonic generation to determine the optical non-linearity is described for materials highly absorptive at the harmonic wavelength. We evaluated the molecular non-linear optical coefficients of nitro compounds and aldehydes, barbituric acid derivatives, fulvene derivatives, chalcone derivatives, retinal derivatives, Schiff base compounds, and fused-ring molecules, at 1.064 µm. Among these materials, nitro molecules and Schiff base compounds had large optical non-linearities and good transparency at visible wavelengths. Barbituric acid derivatives and retinal derivatives also had large non-linearities, but displayed strong absorption of the harmonics. A large enhancement of non-linearities was achieved in fulvene derivatives by extending the conjugated chains. We found that sulfur atoms, non-benzoic conjugated systems and fused rings contributed to non-linear optical effects. In this work, we found that several molecules had very large second-order optical non-linearities; the products of the molecular non-linearities and permanent dipole moments µ₀β of these were of the order of 10^–45–10^–44 esu. The relations between molecular non-linearity and absorption peak energy are discussed.