Routing Of Terahertz Waves By Periodically Structured Metallic Or Dielectric Films

The routing of terahertz waves by periodically structured metallic or dielectric films is investigated theoretically. Spoof surface plasmon polaritons (SPPs) on metal films with an array of periodic cut-through slits show novel properties because of the coupling between the electromagnetic fields on the two surfaces of the film. There exist a series of SPP modes on such structures and the single-mode propagation is available for each mode. For the symmetric structure with the superstate and substrate of the same dielectric, the spoof SPPs can be decomposed into two sets: the symmetric mode whose magnetic field has a local maximum at the center of the metal slits, and the antisymmetric mode whose magnetic field has a local minimum there. The propagation length of the spoof SPPs can be greatly increased by stuffing the slits with silicon slab thicker than the metal film. In this way we can obtain long range propagation with subwavelength confinement, especially for the antisymmetric modes. Spoof SPPs can exhibit lateral confinement when the (lateral) length of the slits in the former structure is changed to be finite. It is shown that by optimizing the slit length, the subwavelength confinement can be achieved in the lateral direction together with long range propagation. Such structure with finite slit length can be used to design subwavelength surface waveguides, and the bend loss of such a waveguide is found to be low. In addition, periodic dielectric waveguide is shown to support backward wave propagation, whose power flow and phase velocity are anti-parallel. The direction-selective coupler can be realized through the coupling between a periodic dielectric waveguide and a conventional dielectric waveguide, whose coupling direction is dependent on the operating frequency.