Experimental Studies of Second Order Nonlinear Optical Susceptibilities in Organic Systems.

Molecular crystals composed of conjugated organic molecular sites exhibit exceptionally large second order optical susceptibilities. The origin of these unusual susceptibilities has been traced to the contribution of highly charge separated excited states in the constituent molecules by successful calculations of these molecular susceptibilities using molecular Hartree-Fock theory. This important theoretical advance required reliable measurements of these molecular susceptibilities. These experimental results together with the theoretical calculations lead to a deeper understanding of the valence electronic excitations in conjugated organic molecular systems through their contribution to charge separated excited states. In turn, the development of new organic and polymeric nonlinear optical materials important to technology is significantly advanced. In this study, a DC induced second harmonic generation (DC-SHG) procedure has been developed to measure the molecular second order susceptibility, (beta), in solutions. Measurements of nitrobenzene, a fundamentally important conjugated molecule, has led to an understanding of the role of local fields and interactions on (beta). This method minimizes the effect of like-species interactions, and with an appropriate solvent, leads to the best experimentally accessible value for (beta). The value for nitrobenzene measured in hexane solutions is (beta) = 1.11 (+OR-) .17 x 10('-30) esu. This procedure was used in studying members of three important new classes of conjugated molecules possessing unusually large second order nonlinear optical susceptibilities. These classes are benzene derivatives, quinoids, and diacetylenes. The observed second order optical susceptibilities were consistent with the results of molecular Hartree-Fock calculations including the dispersion exhibited by solvent shifts of the first electronic excited state making the measurement of (beta) using this procedure a sensitive probe of these valence electrons. Finally, powder measurements of second harmonic generation of crystals composed of members of these three molecular classes are presented. These studies led directly to the discovery of several highly efficient phase matched crystals in each class. The diacetylenes can be polymerized in the solid state leading to high strength polymer single crystals possessing three dimensional long range order. Phase -matched second harmonic generation was observed in an entire class of diacetylene polymer crystals, one example of which is NTDA. It was found that the natural optical anisotropy caused by polymerization of diacetylene monomer crystals enhance phase-matching of the second harmonic signal.