Plasmonic devices for manipulating light at the nanoscale: slow-light waveguides and compact couplers

Abstract

In this dissertation, I explore new plasmonic structures and devices for manipulating light at the nanoscale: slow-light waveguides and compact couplers. I first introduce a plasmonic waveguide system, based on a plasmonic analogue of electromagnetically induced transparency (EIT), which supports a subwavelength slow-light mode, and exhibits a small group velocity dispersion. The system consists of a periodic array of two metal-dielectric-metal (MDM) stub resonators side-coupled to a MDM waveguide. Decreasing the frequency spacing between the two resonances increases the slowdown factor and decreases the bandwidth of the slow-light band. I also show that there is a trade-off between the slowdown factor and the propagation length of the slow-light mode. I next consider Mach-Zehnder interferometer (MZI) sensors in which the sensing arm consists of a slow-light waveguide based on a plasmonic analogue of EIT. I show that a MZI sensor using such a waveguide leads to approximately an order of magnitude enhancement in the refractive index sensitivity, and therefore in the minimum detectable refractive index change, compared to a MZI sensor using a conventional MDM waveguide. I also introduce compact wavelength-scale slit-based structures for coupling free space light into MDM subwavelength plasmonic waveguides. I first show that for a single slit structure the coupling efficiency is limited by a trade-off between the light power coupled into the slit, and the transmission of the slit-MDM waveguide junction. I next consider a two-section slit structure, and show that for such a structure the upper slit section enhances the coupling of the incident light into the lower slit section. The optimized two-section slit structure results in ¡­ 2.3 times enhancement of the coupling into the MDM plasmonic waveguide compared to the optimized single-slit structure. I finally consider a symmetric double-slit structure, and show that for such a structure the surface plasmons excited at the metal-air interfaces are partially coupled into the slits. Thus, the coupling of the incident light into the slits is greatly enhanced, and the optimized double-slit structure results in ¡­ 3.3 times coupling enhancement compared to the optimized single-slit structure. In all cases the coupler response is broadband.