Octupolar versus dipolar crystalline structures for nonlinear optics: A dual crystal and propagative engineering approach

A new type of crystalline structure for nonlinear optics whereby octupolar symmetry features are displayed at both molecular and crystalline levels is exemplified by the prototype 2,4,6-triaryloxy-1,3,5-triazine (TPOT) crystal and analyzed in terms of both individual molecular responses and crystal packing features. Polarized harmonic light scattering permits the full determination of the molecular β hyperpolarizability tensor and confirms the octupolar trigonal symmetry of the TPOT molecule. An oriented gas model is used to infer therefrom an estimate of the crystalline nonlinear d tensor which is predicted to be of the same order as that of the reference dipolar N-4-nitrophenyl-(L)-prolinol crystal. The concept of optimal packing toward quadratic nonlinear optics, which had been initially introduced in the realm of quasi-one-dimensional structures, is revisited and enlarged to encompass more isotropic uniaxial structures potentially amenable, in the case of octupoles, to larger optimal values than in the one-dimensional case. Moreover, considerations pertaining to phase matching which had been left aside in the earlier one-dimensional optimization framework are now considered and the various type I and type II configurations compared for both one-dimensional and octupolar uniaxial structures. Application perspectives of octupolar structures toward short pulse nonlinear optics are discussed: their structurally built-in polarization independence is outlined as a major asset in contrast with the more traditional one-dimensional structures.