  We present here a model for the coupling among small nanoparticles excited by
an optical electric field in the framework of our optical lumped nanocircuit
theory [N. Engheta, A. Salandrino, and A. Alu Phys. Rev. Lett. 95, 095504
(2005)]. We derive how this coupling affects the corresponding nanocircuit
model by adding controlled sources that depend on the optical voltages applied
on the coupled particles. With the same technique, we can model also the
presence of a substrate underneath nanocircuit elements, relating its presence
to the coupling with a properly modeled image nanoparticle. These results are
of importance in the understanding and the design of complex optical
nanocircuits at infrared and optical frequencies.
