Size-Dependent Validity Bounds on the Universal Plasmon Ruler for Metal Nanostructure Dimers

We study the validity of the recently proposed universal plasmon ruler in the present work using a combination of numerical techniques based on the finite difference time domain (FDTD) method, and semianalytical theories based on the coupled dipole approximation. By incorporating nonlocal effects for closely spaced two-dimensional gold nanocylinder dimers, we find using both the FDTD and semianalytical approaches that the universal plasmon ruler of Jain et al. is not applicable for gold nanocylinder dimers with diameters smaller than about 20 nm. The nonlocal effects are also found to strongly reduce the electric field enhancements at very small gap distances. Taken together with previous results, we are able to establish the valid size range for the universal plasmon ruler of gold: 20 nm diameter metal nanostructure dimers at the smaller end, and 70 nm diameter metal nanostructure dimers at the larger end.