Third-order nonlinear optical properties of dendritic molecular aggregates: Effects of fractal architecture

摘要

We investigate the microscopic third-order nonlinear optical properties, i.e., the second hyperpolarizabilities (gamma), of two different sizes of molecular aggregates with a dendritic, i.e., Bethe-lattice, structure. One possesses a nonfractal structure, while the other has a fractal structure. The aggregate is treated in a two-exciton model composed of two-state monomers coupled to each other by the dipole-dipole interaction. The off-resonant gamma of the aggregates are calculated by the numerical Liouville approach, including relaxation effects. The total gamma value is partitioned into the contribution of virtual exciton generation, and its spatial contribution to gamma is analyzed in relation to the virtual excitation processes in the perturbation theory. It is found that the intermolecular-interaction effect enhances both one- and two-exciton-generation contributions, while the: relaxation effect reduces those, although the one- and two-exciton-generation contributions have mutually opposite signs. From the comparison of spatial contributions to gamma between the nonfractal and fractal aggregates, an enhancement of the contribution to gamma from the periphery to the core is observed in the fractal structure, while such a feature is not observed in the nonfractal structure. (C) 2001 John Wiley & Sons, Inc. [References: 18]