Light Coupling Structures And Switches For Plasmonic Coaxial Waveguides

We introduce wavelength-scale light coupling structures and switches for plasmonic coaxial waveguides. We first consider single-slit structures optimized for a wavelength of 1550nm and find that, when the slit is on resonance, the coupling to the plasmonic coaxial waveguide is maximized. We also observe that for optimized double- and triple-slit structures, the coupling efficiency is enhanced compared to the single-slit structure by factors of similar to 3.02 and similar to 4.21, respectively. We find that, in the case of double- and triple-slit structures, the surface plasmons excited at the metal-air interface enhance light coupling to the plasmonic coaxial waveguide via the slits. In addition, we investigate slit-based outcoupling structures for light extraction from the waveguide into a free space. We observe that while the far-field radiation pattern of single-slit structures is symmetric, double- and triple-slit structures have asymmetric radiation patterns. We also show that by exciting the incoupling slit structures at proper angles, we can excite only the right- or the left-propagating mode of the plasmonic coaxial waveguide. We finally design compact plasmonic switches consisting of a plasmonic coaxial waveguide side-coupled to a periodic array of two open-circuited coaxial stub resonators. Such a structure is based on a plasmonic analog of electromagnetically induced transparency and supports a slow-light mode. The space between the metallic parts is filled with an active material with a tunable refractive index. We show that the modulation depth of this structure is large enough for optical switching applications.