Second Order nonlinear optical properties of the single crystal of N-benzyl 2-methyl-4-nitroaniline: Anomalous enhancement of the d₃₃₃ component and its possible origin

A single crystal of N-benzyl 2-methyl-4-nitroaniline (BNA) whose dimensions were 8 × 10 mm (diameter × length) was grown using a vertical Bridgman method with sufficient optical transparency for spectroscopic use. Polarized reflection spectra of the crystal were measured in order to determine its linear optical parameters (refractive indices and extinction coefficients). The relative magnitudes of the d-tensor components of the crystal were determined to be d₃₃₃ = (17 ± 9) × d₃₂₂, d₃₂₂ > d₃₁₁, d₃₁₁ ∼ 0 from polarization dependencies of the second harmonic generation. These results are in disagreement with previously reported theoretical studies, in which the effects of intermolecular interactions were completely neglected. Here, we have included these intermolecular interactions in the calculations assuming the presence of supra-molecular clusters. We conclude that the anomalous enhancement of d ₃₃₃ is due to the interactions along the direction of the hydrogen bonds in the crystal and the resonance effect.