Negative effective permeability in metal cluster photonic crystal

We report a new metamaterial design made of a periodic array of metal nanowire clusters. For transverse-electric polarization, the metal nanowire supports an electric-dipole-like Mie resonance. When the nanowires are arranged into a regular array with sufficiently small spacing, the array exhibits a resonant behavior in effective permittivity. Furthermore, when the nanowires are arranged into a finite size cluster, they can support a magnetic Mie resonance in which magnetic field is strongly localized inside the cluster. Array of such clusters with sufficiently small spacing can then exhibit a resonant behavior in effective permeability. When the magnetic resonance is strong enough, permeability can become negative. The mechanism of producing negative permeability is similar to the ferroelectric and polaritonic photonic crystals but the metal cluster photonic crystal can exhibit stronger magnetic activity at optical frequencies. The availability of extensive synthesis and fabrication techniques for metal nanostructures makes the metal cluster photonic crystal a promising metamaterial platform for optical frequency operation.