Guided subwavelength optical mode with slow group velocity supported by a periodic plasmonic waveguide

We introduce a periodic plasmonic waveguiding structure which supports a guided subwavelength optical mode with slow group velocity at a tunable wavelength range and with a tunable slowdown factor. The structure consists of a metal-dielectric-metal (MDM) waveguide side-coupled to a periodic array of MDM stub resonators. Both the MDM waveguide and MDM stub resonators have deep subwavelength widths. We show that such a structure supports a guided optical mode with slow group velocity. The wavelength range in which slow light propagation is achieved can be tuned by adjusting the MDM stub resonator length and the periodicity of the structure. We also show that the slowdown factor increases as the periodicity of the structure decreases, and that light can be slowed down by several orders of magnitude. We find that there is a tradeoff between the slowdown factor and the propagation length of the supported optical mode. In addition, for a given slowdown factor and operating wavelength, the propagation length of the optical mode in the periodic plasmonic waveguide is much larger than the propagation length of the mode supported by a conventional MDM waveguide, in which the slowdown factor can be tuned by adjusting the dielectric layer width. Finally, we show that light can be coupled efficiently from a conventional MDM waveguide to such a periodic plasmonic waveguide. Such slow-light plasmonic waveguides could be potentially used in nonlinear and sensing applications. We use a characteristic impedance model and transmission line theory to account for their behavior.