Quadratic Nonlinear Optical Susceptibilities of a New SHG Material, 3-Aminoxanthone: A Theoretical Study on Its Molecular and Crystal Susceptibilities

Nonlinear optical susceptibilities of a new SHG (second harmonic generation) material, 3-aminoxanthone, were investigated at both molecular and crystalline levels, by using the ab initio CPHF (coupled perturbed Hartree?Fock) method and the oriented-gas approximation. Molecular first hyperpolarizabilities β of 3-aminoxanthone were analyzed in order to investigate the source of β, by using an intramolecular oriented-gas approach which assumes the molecular β can be obtained by making a tensor sum of the β of molecular fragments. The analysis revealed the source of β is localized on one aromatic ring, and effective π-conjugation for β is almost completely disconnected between the aromatic rings. The results of the oriented-gas calculations showed the crystal has a phase matching condition of SHG, and its is 10.6 times that of the of urea crystal; these results are in agreement with the results of the SHG powder efficiency measurements. Experimental refractive indices were needed for the local field correction of the oriented-gas approximation in order to estimate the experimental order of magnitude of χ⁽²⁾. The localized source of β may provide a new way for molecular design of organic nonlinear optical materials.