Off-resonance field enhancement by spherical nanoshells

We study light scattering by spherical nanoshells consisting of metal-dielectric composites. We consider several geometries of nanoshell particles with dielectric and metallic cores. We demonstrate that in all studied cases the local field enhancement takes place in out-of-resonance regions ("dark states"), which nevertheless can be understood in terms of the Fano resonance. At optimal conditions the light is mostly localized inside the dielectric material and the enhancement factor up to four orders of magnitude is achieved for subwavelength coating. Using nonlinear dielectric materials will lead to a variety of nonlinear phenomena applicable for photonics applications.