Active plasmonic devices enhanced by waveguide dispersion engineering

Abstract

Plasmonic devices, based on surface plasmons propagating at metal-dielectric interfaces, have shown the potential to manipulate light at deep subwavelength scales. One of the main challenges in plasmonics is achieving active control of optical signals. In this paper, we introduce active plasmonic devices enhanced by waveguide dispersion engineering. We consider plasmonic waveguide systems consisting of a metal-dielectric-metal waveguide (MDM) side-coupled to arrays of MDM stub resonators. The MDM waveguide and stubs are filled with an active material whose absorption coefficient can be modified with an external control beam. Such plasmonic waveguide systems can be engineered to support slowlight modes. We find that, as the slowdown factor increases, the sensitivity of the effective index of the mode to variations of the refractive index of the active material increases. Such slow-light enhancements of the sensitivity to refractive index variations lead to enhanced performance of active plasmonic devices such as switches. To demonstrate this, we consider absorption switches based on Fabry-Perot cavity structures, consisting of slow-light plasmonic waveguide systems sandwiched between two conventional MDM waveguides. We find that increased slowdown factor leads to increased induced change of the propagation length of the slow-light mode for a given refractive index variation, and therefore to increased modulation depth. Compared to conventional MDM absorption switches, slow-light enhanced switches achieve significantly higher modulation depth with moderate insertion loss. We use a scattering matrix theory to account for the behavior of the devices which is in excellent agreement with numerical results obtained with the finitedifference frequency-domain method.