  We present an experimental and theoretical study of the optical properties of
metal-dielectric-metal structures with patterned top metallic surfaces, in the
THz frequency range. When the thickness of the dielectric slab is very small
with respect to the wavelength, these structures are able to support strongly
localized electromagnetic modes, concentrated in the subwavelength metal-metal
regions. We provide a detailed analysis of the physical mechanisms which give
rise to these photonic modes. Furthermore, our model quantitatively predicts
the resonance positions and their coupling to free space photons. We
demonstrate that these structures provide an efficient and controllable way to
convert the energy of far field propagating waves into near field energy.
