Interactions of Nanorod Particles in the Strong Coupling Regime

The plasmon coupling in a nanorod dimer obeys the exponential size dependence according to the universal plasmon ruler equation. However, it was shown recently that such a model does not hold at short nanorod distances (Nano Lett. 2009, 9, 1651). Here we study nanorod coupling in various cases, including a nanorod dimer with asymmetrical lengths and a symmetrical dimer with varying gap width. The asymmetrical nanorod dimer causes two plasmon modes: one is the attractive lower energy mode and the other the repulsive high-energy mode. Using a simple coupled LC-resonator model, the position of dimer resonance has been determined analytically. Moreover, we found that the plasmon coupling of a symmetrical cylindrical (or rectangular) nanorod dimer is governed uniquely by the gap width scaled for the (effective) rod radius rather than for the rod length. A new plasmon ruler equation without using the fitting parameters has been proposed which agrees well with the finite-difference time-domain calculations. The method has also been extended to study plasmonic waveguiding in a linear chain of gold nanorod particles. A field decay length up to 2700 nm with a lateral mode size of about 50 nm (similar to lambda/28) has been suggested.