Tuning Fluorescence Direction with Plasmonic Metal-Dielectric- Metal Substrates

Abstract

Controlling the emission properties of fluorophores is essential for improving the performance of fluorescence-based techniques in modern biochemical research, medical diagnosis, and sensing. Fluorescence emission is isotropic in nature, which makes it difficult to capture more than a small fraction of the total emission. Metal- dielectric-metal (MDM) substrates, discussed in this Letter, convert isotropic fluorescence into beaming emission normal to the substrate. This improves fluorescence collection efficiency and also opens up new avenues for a wide range of fluorescence-based applications. We suggest that MDM substrates can be readily adapted for multiple uses, such as in microarray formats, for directional fluorescence studies of multiple probes or for molecule-specific sensing with a high degree of spatial control over the fluorescence emission.

Section: Physical Processes in Nanomaterials and Nanostructures.

Figure 1

Angular distributions of the fluorescence intensities for s- (black) and p-polarized (red) emission of S101 embedded in Ag-PVA-Ag substrates with different PVA concentrations. The calibration curve for the PVA film thickness obtained with varying PVA concentrations is provided in the Supporting Information.

Figure 2

Calculated reflectivity curves for 600 nm s- (black) and p-polarized (red) light for (A) Ag-PVA(4%)-Ag, PVA thickness 150 nm and (B) Ag-PVA(8%)-Ag, PVA thickness 350 nm.

Figure 3

Emission spectra of S101 (s-polarized) from the PVA(4%)-Ag substrate recorded at 125, 130, 132, 133, 134, 135, 136, and 140° (1–8) and the Ag-PVA(4%)-Ag substrate recorded at 125, 130, 140, 150, 160, 170, and 180° (1–7).

Figure 4

Angular distribution for emission at 600 nm (S101) and 670 nm (Cy5) for a mixed sample of S101 and Cy5 in Ag-PVA(6%)-Ag. Right panel shows emission spectra recorded for the mixed sample at 145 and 180° observation angles.

Figure 5

Illumination scheme and superimposed photographs of the fluorescence images of S101 projected on a screen for each excitation spot in (A) PVA-Ag and (B) Ag-PVA-Ag.

Figure 6

Fluorescence spectra and intensity decays (inset) for (1) donor only, AlQ₃/PMMA(4%)-Ag-PVA(4%)-Ag, (2) acceptor only, PMMA(4%)-Ag-PVA(4%)/Rh6G-Ag, and (3) donor–acceptor, AlQ₃/ PMMA(4%)-Ag-PVA(4%)/Rh6G-Ag samples with 470 nm excitation and observed at 180°. Top panel shows schematic of the SPP-ET process.

Scheme 1. Illumination Geometry (Reverse Kretschmann, RK) and Angle Notations Used in the Study^a

^aAg-PVA-Ag substrate is prepared on a glass slide and fixed to the prism with glycerol. The s- and p-polarizations are shown for clarity