Plasmon-enhanced total-internal-reflection fluorescence by momentum-mismatched surface nanostructures.

We investigate optimum plasmon-enhanced total-internal-reflection fluorescence imaging by metallic thin films and nanostructures. The enhancement is based on the mismatch between the conditions of plasmon resonance and maximal near-field intensity. We have calculated plasmon-associated near-field and far-field characteristics using rigorous coupled-wave analysis. Near-field intensity was experimentally measured with fluorescent beads on silver thin films, nanogratings, and nanoislands. The results for nanostructure-based plasmon excitation confirm that momentum mismatching when exciting plasmons can increase the consequent emission of fluorescence substantially. The improvement can be critical depending on the specific structure.