Plasmon- and Waveguide-Coupled Fluorescence at the Ultraviolet Region.

Surface plasmon-coupled emission (SPCE) has been well studied for its coupled, directional, and enhanced P-polarized radiation due to the interactions of fluorophores with surface plasmon polaritons (SPPs) on thin metal films. Such surface plasmon polariton-assisted directional fluorescence has various applications in biosensing. Herein, we demonstrate 2-aminopurine (2AP, a UV-absorbing and -emitting fluorophore) emission coupling to modes in aluminum-based plasmon-coupled waveguides (Al-PCWs). Directional emission from 2-aminopurine on plasmon-coupled waveguides was observed at specific angles as P-polarized SPCE and/or as P- or S-polarized waveguide-coupled emission (WGCE). All S-polarized waveguide modes showed clear angularly resolved emission as compared to that of P-polarized surface plasmon-coupled emission or P-polarized waveguide-coupled emission. The coupling angles, efficiencies, and polarizations of the modes were sensitive to the optical properties and overall dimensions of the top dielectric layer in PCWs. The effective plasmon-coupled waveguide can consist of either a thin probe-containing layer on top of the undoped silica film, or a single dielectric PVA layer with probes distributed throughout the film on the Al layer. The former structures with probes confined to the top of the undoped silica layer showed much higher angular resolutions and coupling efficiencies, as well as mode-dependent changes in lifetimes. These results demonstrate that the plasmon and waveguide modes can be used for selective detection of surface-bound and bulk fluorophores, simultaneously.