  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 distance (Nano Lett.
2009, 9, 1651). Here we study the nanorod coupling in various cases, including
nanorod dimer with the asymmetrical lengths and symmetrical dimer with the
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 symmetrical cylindrical (or rectangular) nanorod dimer is governed
uniquely by 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 FDTD calculations. The method has
also been extended to study the plasmonic wave-guiding in a linear chain of
gold nanorod particles. A field decay length up to 2700nm with the lateral mode
size about 50nm (~wavelength/28) has been suggested.
