  Hybrid metallic nanoparticles encapsulated in oxide shells are currently
intensely studied for plasmonic applications in sensing, medicine, catalysis,
and photovoltaics. Here, we introduce a method for the synthesis of
Au@Ag@SiO$_2$ cubes with a uniform silica shell of variable and adjustable
thickness in the nanometer range; and we demonstrate their excellent, highly
reproducible, and tunable optical response. Varying the silica shell thickness,
we could tune the excitation energies of the single nanoparticle plasmon modes
in a broad spectral range between 2.55 and 3.25\,eV. Most importantly, we
reveal a strong coherent coupling of the surface plasmons at the silver-silica
interface with the whispering gallery resonance at the silica-vacuum interface
leading to a significant field enhancement at the encapsulated nanoparticle
surface in the range of 100\,\% at shell thicknesses $t\,$$\simeq\,$20\,nm.
Consequently, the synthesis method and the field enhancement open pathways to a
widespread use of silver nanoparticles in plasmonic applications including
photonic crystals and may be transferred to other non-precious metals.
