  The theory and design of superbackscattering nanoparticle dimers are
presented. We analytically derive the optimal configurations and the upper
bound of their backscattering cross-sections. In particular, it is demonstrated
that electrically small nanoparticle dimers can enhance the backscattering by a
factor of 6.25 with respect to single dipolar particles. We demonstrate that
optimal designs approaching this theoretical limit can be found by using a
simple circuit model. The study of practical implementations based on plasmonic
and high-permittivity particles reveal that fourfold enhancement factors might
be attainable even with realistic losses.
