Abstract

The field of Molecular Nonlinear Optics¹ has been seeded in the mid-seventies by the all pervading molecular diode template as exemplified by paranitroaniline derivatives exhibiting the now well recognized features of a rod-like dipolar structure with quasi-one dimensional intramolecular charge transfer between a single donor-acceptor pair via a delocalized π electron linkage. A highly consistent body of experiments in solutions, crystals and polymers as well as related models have come to reinforce the validity of this approach which has been further substantiated by a series of conceptual as well as application oriented developments. NLO measurements are now well acknowledged and currently used in Chemistry as sensitive probes of such basic features as intramolecular charge transfer or π-electron delocalization in conjugated systems whereas recent demonstrations of high bandwidth electrooptic modulation in polymer based modulators² or low threshold near IR infrared parametric oscillation in molecular crystals³ evidence the application potential of the field. The existence of a large ground state dipole moment and the possibility to couple it to a poling electric field via a Langevin-Boltzmann distribution stands-out as the major cornerstone responsible for the development of the field. A two-level model pointing-out the difference between excited and ground state dipoles as a basic underlying parameter further strengthens the relevance of a dipolar geometry. However, such highly consistent interplay of dipolar models, experiments and technologies may have lead to ignore broader possibilities as recently recognized in the form of octupolar (see Fig.1) and more generally, multipolar systems⁴.

© 1996 Optical Society of America