Third-order nonlinear optical properties in Langmuir–Blodgett films of 4-docosylamino-4-nitroazobenzene

Abstract
The third-order nonlinear optical properties of 4-docosylamino-4-nitroazobenzene (DANAB) Langmuir–Blodgett (LB) films were investigated. The orientation of the DANAB molecules in the LB film is designed to be aligned virtually perpendicular to the substrate by mixing DANAB and fatty acid, which has the effect of preventing the cascading effect. It was found that DANAB film, mixed with a fatty acid, form H aggregates. The frequency dispersion was measured using the third harmonic generation method with the optical parametric oscillator. It is indicated that the third-order nonlinear optical susceptibility [χ(3)] is enhanced because of the resonance and molecular aggregation effects. The value of the χ(3) under the resonant condition of 3.1×10−12 esu and that under the nonresonant condition of 1.5×10−12 esu were obtained.