Second-harmonic generation due to coulomb-like interaction in a heterodimer of subwavelength dimensions

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.