Light trapping beyond the 4<i>n</i>2 limit in thin waveguides

Abstract

We describe a method for determining the maximum absorption enhancement in thin film waveguides based on optical dispersion relations. For thin film structures that support one, well-confined guided mode, we find that the absorption enhancement can surpass the traditional limit of 4n2 when the propagation constant is large and/or the modal group velocity is small compared to the bulk value. We use this relationship as a guide to predicting structures that can exceed the 4n2 light trapping limit, such as plasmonic and slot waveguides. Finally, we calculate the overall absorption for both single and multimode waveguides, and show examples of absorption enhancements in excess of 4n2 for both cases.