Surface Plasmon-Coupled Emission with Gold Films

Abstract

In a recent report we demonstrated efficient collection of emission by coupling to surface plasmons on a thin silver film, resulting in a directional signal in the glass substrate. We call the phenomenon surface plasmon coupled emission (SPCE). In the present report we examined sulforhodamine 101 (S101) in thin polymer films on 50 nm thick gold films on glass. We observed efficient SPCE through thin gold films. This result was surprising because metallic gold is typically an efficient quencher of fluorescence. The energy effectively coupled through the gold film into the glass at a sharply defined angle, but somewhat less sharp than for a comparable silver film. About 50% of the total emission appeared as SPCE, irrespective of direct excitation or excitation via the plasmon resonance evanescent wave. The emission was p-polarized with different wavelengths appearing at different angles. The lifetime of S101 was mostly unaffected by the gold film. These results indicate that SPCE occurs over long distances, larger than for quenching by energy transfer to the gold. We conclude highly efficient detection devices can be constructed by using fluorophores on gold-coated surfaces.

Figure 1

Angular distribution of the emission of S101 in PVA (40 nm thick) on a gold film (top) and a silver film (bottom). The excitation was horizontally polarized and the emission observed through a horizontal polarizer.

Figure 2

Emission spectra of S101 in PVA on a gold film measured with the RK configuration. V and H represent vertical and horizontal polarization in the order excitation–emission. Top panel: p-polarized excitation. Bottom panel: s-polarized excitation.

Figure 3

Calculated reflections for gold (top) and silver (bottom) mirrors. The coupling hemicylinder prism was from BK7 glass, n = 1.52; the d₁ layer was PVA, n = 1.50. For silver we used ε = −17 + 0.6i and for gold we used −8.9 + 1.07i.

Figure 4

Emission spectra of S101 in PVA on gold recorded with different observation angles.

Figure 5

Photographs of the S101 directional emission in PVA on gold mirror side-view (top) and front view (middle). Bottom: emission cone from S101 on a silver film.

Figure 6

Effect of the excitation incidence angle for the free space emission intensity of S101 in PVA on gold (top) and silver (bottom).

Figure 7

Angular distribution of the S101 emission in PVA on gold (top) and silver (bottom) observed with KR (SP) excitation.

Figure 8

Emission spectra of S101 in PVA on gold (top) and silver (bottom) with p-polarized SP excitation observed through horizontally (−) or vertically (- - -) oriented polarizers.

Figure 9

Frequency-domain intensity decays of S101 in PVA on gold. Top and middle, RK excitation. Top: free-space emission. Middle: surface plasmon-coupled emission. Bottom: KR excitation and surface plasmon-coupled emission.

SCHEME 1

Geometry for SPCE Measurements with the KR Configuration (top) and for SPCE Measurements with the RK Configuration (bottom)

SCHEME 2

Schematic of the Cone of Emission in RK Configuration