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.

Automated summary

This study introduces wavelength-scale light coupling structures and switches for plasmonic coaxial waveguides. By optimizing single-slit, double-slit, and triple-slit structures, the coupling efficiency is enhanced, and different propagation modes can be generated. Additionally, the study explores outcoupling structures for light extraction and utilizing proper angle excitation for specific modes in plasmonic coaxial waveguides.