Effects of symmetry breaking and conductive contact on the plasmon coupling in gold nanorod dimers.

We have explored the consequences of symmetry breaking on the coupled surface plasmon resonances in individual dimers of gold nanorods using single-particle dark-field scattering spectroscopy and numerical simulations. Pairs of chemically grown nanorods can exhibit wide variation in sizes, gap distances, and relative orientation angles. The combination of single-particle spectroscopy and theoretical analysis allowed us to discern the effects of specific asymmetry-inducing parameters one at a time. The dominant influence of symmetry breaking occurred for longitudinal resonances in strongly coupled nanorods in linear end-to-end configurations. In particular, we found that the normally dark antibonding dimer mode becomes visible when the sizes of the two nanorods are different. In addition, we observed a conductively coupled plasmon mode that was red-shifted by at least 250 nm from the bonding plasmon mode for the corresponding nontouching geometry. Gaining detailed insight into how symmetry breaking influences coupled surface plasmon resonances of individual nanorod dimers is an important step toward the general understanding of the optical properties of assemblies of chemically synthesized nanorods with unavoidable irregularities in size and orientation.