Design of Asymmetrical Grating Couplers for Surface Plasmon Enhanced Illuminators through Rigorous Computation

Finite difference time domain electromagnetic simulation of asymmetrical grating structures is used to identify promising plasmon enhanced illuminators (PEI). Such PEIs have important applications in microscopy, surface enhanced Raman scattering, and even heat-assisted magnetic recording. The design of PEIs is complicated by the need for directional coupling and low re-radiation of previously coupled energy. Computer simulation can guide this technology development by giving insight to the physical interaction and quantitative coupling coefficients. Our approach is to use finite-difference time domain analysis to explore coupling efficiency as a two step process. The first step is the characterization of the scattering from collection elements. The second step is to combine elements using a signal flow graph based on two port models. Of particular interest are asymmetrical elements that in addition to providing directional coupling, also allow low re-radiation. Ultimately, through this approach we aim to increase the local electric field transmission by a factor of 1,000 for surface plasmon enhanced Raman scattering. The two step process will be verified by direct computation of the full grating geometry.