Orientation Filtering of Fluorescence by Plasmon-Coupled Waveguide

Abstract

Plasmon-coupled waveguide (PCW) is an interesting structure that combines plasmons with dielectric waveguides to achieve dual polarization and improved properties of optical modes that are beneficial for fluorescence modulations. This study investigates the effect of emission dipole orientations on the mode-coupled emission from fluorophores doped on a PCW-substrate that is designed to support one S-and one P-polarized waveguide (WG) mode. Differently oriented excited populations of the fluorophore, sulforhodamine 101 (S101), have been generated by the principle of photoselection, using horizontally or vertically polarized excitation light. Further, two different excitation wavelengths have been chosen to access collinear and noncollinear excitation/emission dipoles. Variations in angular fluorescence intensity distribution patterns of the WGcoupled emissions of S101 reveal that coupling of radiated power with the two coexisting orthogonally polarized modes in the PCW-substrate depends critically on the orientation of emission dipoles. The orientation ratio (R), which is defined as a measure of the difference in the intensity ratio between S-and P-polarized coupled emissions for horizontal and vertical excitations, is found to be positive for collinear excitation/emission transition dipoles and negative for noncollinear excitation/emission transition dipoles. The orientation-based filtering of emission by PCW-substrates is not only of interest for display applications and light-emitting devices but can also be useful for deciphering the orientations of molecules doped in the substrates.