We present an experimental and theoretical study of the optical properties ofmetal-dielectric-metal structures with patterned top metallic surfaces, in theTHz frequency range. When the thickness of the dielectric slab is very smallwith respect to the wavelength, these structures are able to support stronglylocalized electromagnetic modes, concentrated in the subwavelength metal-metalregions. We provide a detailed analysis of the physical mechanisms which giverise to these photonic modes. Furthermore, our model quantitatively predictsthe resonance positions and their coupling to free space photons. Wedemonstrate that these structures provide an efficient and controllable way toconvert the energy of far field propagating waves into near field energy.
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