Recently, the preparation of core-shell colloids comprising a dielectric solid sphere (e.g., silica, poly(styrene)) covered with a continuous metallic shell has attracted much attention. Such interest stems from he unique optical properties exhibited by these colloids as either single entities in solution (multipole plasmon modes), or when used as functional building blocks for incorporation into microstructures that are useful for coupling with electromagnetic radiation (e.g., "antenna" effect), or organized into colloidal crystals (e.g., photonic bandgap materials). A main characteristic of dielectric spheres with continuous metallic coatings is that their optical plasmon absorption consists of several modes, generally dipole and quadrupole, with the peak positions depending on the dielectric core diameter to metallic shell thickness ratio. Varying this ratio allows tuning of the peak position from the UV to the IR range. Multipole as well as split plasmon modes have been observed for planar nanoparticulate metallic films. For coated spheres, it has been predicted from Mie scattering theory, and corroborated by experimental data, that control over the thickness of the metallic shell constitutes a simple alternative to tune the optical properties with spectrally isolable multipole modes. However, no splitting of the plasmon modes has been observed for such colloids.
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