Metaparticles: Dressing Nano‐Objects with a Hyperbolic Coating

The ability to engineer the optical response of a plasmonic nano‐object is highly desired to achieve better control over light–matter interactions. Due to the sensitivity of plasmon resonances to the surrounding media, isotropic dielectric coating is an easy approach to modify the optical properties of a plasmonic nanostructure. However, the choice of coatings and the provided tunability is limited by the range of refractive indices of available materials. Here, it is shown that coating of plasmonic nano‐objects with an anisotropic metamaterial, which displays a hyperbolic dispersion and allows the design of refractive index on demand, provides greater flexibility in engineering their interaction with light. This is experimentally demonstrated by coating Au nanospheres with alternating SiO2 and Au multishells. This creates rich and highly tunable plasmonic modes covering a broad wavelength range (≈400–2200 nm) and produces high local field intensity enhancement (≈500‐fold). The concept is extended to hyperbolic coating of dielectric nano‐objects, confirming the nature of the modes to be related to the resonances in the hyperbolic layer. The implemented approach using a coating with an engineered effective refractive index may find applications in plasmon‐enhanced spectroscopy, nanolasers, design of nonlinear phenomena, photothermal conversions, and hot‐electron generation.