  Localized plasmonic modes of metallic nanoparticles may hybridize like those
of atoms forming a molecule. However, the rapid decay of the plasmonic fields
outside the metal severely limits the range of these interactions to tens of
nanometers. Herein, we demonstrate very strong coupling of nanocavities in
metal films, sparked by propagating surface plasmons and evident even at much
larger distances of hundreds of nanometers for the properly selected
metal/wavelength combination. Such strong coupling drastically changes the
symmetry of the charge distribution around the nanocavities making it amenable
to probing by the nonlinear optical response of the medium. We show that when
strongly coupled, equilateral triangular nanocavities lose their individual
three-fold symmetry to adopt the lower symmetry of the coupled system and then
respond like a single dipolar entity. A quantitative model is suggested for the
transition from individual to strongly coupled nanocavities.
