Generating Localized Plasmonic Fields on an Integrated Photonic Platform using Tapered Couplers for Biosensing Applications

A theoretical design and analysis of a tapered-coupler structure on a silicon nitride integrated-photonic platform for coupling optical energy from a dielectric waveguide to a plasmonic tip is presented. The proposed design can be considered as a hybrid photonic-plasmonic structure that generally supports hybrid symmetric and asymmetric modes. Along the taper, one of the hybrid modes approaches the cut-off, while the other approaches the short-range surface plasmon mode that generates localized fields. Potential use of the proposed novel tapered-coupler plasmonic structure for highly sensitive biosensing applications using surface enhanced Raman scattering (SERS) and metal enhanced fluorescence (MEF) techniques is discussed. For SERS, a theoretical electromagnetic enhancement factor as high as 1.23 × 10(6) is deduced for taper tip widths as small as 20 nm. The proposed tapered-coupler sets up interesting possibilities towards moving to an all-integrated on-chip SERS and MEF based bio-sensor platform - away from traditional free-space based illumination strategies.