  We experimentally study the optical second harmonic generation (SHG) from
deep subwavelength gold-silver heterodimer and silver-silver and gold-gold
homodimers. Our results show that the SHG from the heterodimer is about an
order of magnitude more intense. In contrast, calculations based on known
theory suggest that it is the silver-silver homodimer that should have the
upper hand, putting the present theoretical view at odds with our experimental
findings. Following this observation, we propose a dynamic model where
nonlinearity emerges not from the material particles themselves but form the
Coulombic interaction between them that prevailed over other nonlinear optics
mechanisms at the extreme subwavelength dimensions of the dimer in this case.
The model's good agreement, combined with the failure of the conventional
theoretical view, implies a yet uncharted nonlinear optical effect.
