A broad-band perfect absorber composing a two-dimensional periodic metal-dielectric-metal sandwiches array on dielectric/metal substrate is designed and numerically investigated. It is shown that the nearly-perfect absorption with a bandwidth of about 50 nm in visible region can be achieved by overlapping of two plasmon resonances: one originating from the coupling of electric dipoles between adjacent unit cells and another arising from magnetic dipole plasmon resonances. A capacitor-inductor circuit description is introduced to explain the dependence of resonance frequencies and band-width on geometrical parameters.

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Dual-band perfect absorbers involving an array of gold nanoellipsoids or nanoellipsoid dimers near a gold film are designed and their resonance absorptions and field enhancements are numerically investigated. It is shown that the multilayer structures combining gold nanoparticles and gold film enable dual-band higher absorption and larger field enhancement than the structures without the metal film, due to the strong coupling between the localized surface plasmon resonance from the particles or particle dimers and the surface plasmon polaritons from the gold film. A field enhancement factor (|E|/|E0|) of more than 102 is achieved at the positions of 2.5 nm from the particle tip at two distinct wavelengths in the surface-plasmon-coupled nanoellipsoid dimer structure. The results suggest that the coupling of the localized surface plasmon resonance and the surface plasmon polaritons excited in the plasmonic metamaterials may have potential applications for multifrequency absorbers, emitters and SERS substrates as well as sensing.