The effect of silica coating on the optical response of sub-micrometer gold spheres

The effects derived from the growth of silica shells on the optical properties of sub-micrometer gold spheres have been investigated as a function of shell thickness. The dipole plasmon resonance mode of these large spheres has been found to be significantly more sensitive to changes in the medium refractive index than the quadrupolar mode, in agreement with theoretical modeling based on the boundary element method (BEM) for concentric spheres. Near-field maps of both resonance modes in such coated nanoparticles show that the areas with significant near-field enhancement get progressively located within the silica shell as its thickness increases, with basically no enhancement outside of the shell when this is thicker than the sphere radius.