  The response of gold nanoparticle dimers is studied theoretically near and
beyond the limit where the particles are touching. As the particles approach
each other, a dominant dipole feature is observed that is pushed into the
infrared due to interparticle coupling and that is associated with a large
pileup of induced charge in the interparticle gap. The redshift becomes
singular as the particle separation decreases. The response weakens for very
small separation when the coupling across the interparticle gap becomes so
strong that dipolar oscillations across the pair are inhibited.
Lower-wavelength, higher-order modes show a similar separation dependence in
nearly touching dimers. After touching, singular behavior is observed through
the emergence of a new infrared absorption peak, also accompanied by huge
charge pileup at the interparticle junction, if initial interparticle-contact
is made at a single point. This new mode is distinctly different from the
lowest mode of the separated dimer. When the junction is made by contact
between flat surfaces, charge at the junction is neutralized and mode evolution
is continuous through contact. The calculated singular response explains recent
experiments on metallic nanoparticle dimers and is relevant in the design of
nanoparticle-based sensors and plasmon circuits.
