Multipolar engineering of molecules and materials for quadratic nonlinear optics

The first generation of molecular engineering studies whereby a one-dimensional bipolar donor–acceptor conjugated system had served, since the early 1970s, as a quasi-exclusive template, had failed to take into account the tensorial dimension attached to the β quadratic hyperpolarizability tensor. We review here the outgoing second generation of molecular design studies based on three-dimensional multipolar charge transfer which reflects on enhanced values for all βijk coefficients, then opening-up the possibility of polarization-independent nonlinear interactions. This can be ensured by more isotropic crystalline lattices or statistical polymer-based orientational distribution out of reach for the cruder one-dimensional systems. A particular challenge which has in fact triggered this domain in the early 1990s to this day has been set by octupolar systems whereby the symmetry-imposed cancellation of the dipole moment precludes the utilization of an electric field coupling scheme to orient molecules into a noncentrosymmetric order. We will review this new domain, encompassing both physical and chemical considerations, all the way from new molecular engineering techniques, with associated quantum modeling, to advanced characterization methods and macroscopic ordering schemes jointly aiming at promoting and exploiting the so far largely ignored tensorial dimension of nonlinear light–matter coupling schemes in molecules. To cite this article: I. Ledoux, J. Zyss, C. R. Physique 3 (2002) 407–427.