We measured the emission intensities for all accessible angles around the z-axis in the plane of the Pt films. We used RK excitation since the incident light from this direction cannot excite surface plasmons. shows polar intensity plots showing the SPCE and free space emission of PVA films containing fluorescein (top panel) and rhodamine B (bottom panel) respectively on 40-nm platinum films. The emission is sharply directed back into the hemicylinder and is distributed equally on both sides of the normal axis as shown in . The emission was sharply distributed at 50° and 48° for fluorescein and rhodamine respectively. also shows the highly directional and predominantly p-polarized SPCE emission on Pt films, suggesting that the observed signal is due to surface plasmons. This is in stark contrast to the free-space emission which does not show any polarization or directional preference. It can be seen that the p-polarized signal intensity at the SPCE peak angle is lower in magnitude than the unpolarized signal. This occurs because the sheet polarizers used in the experiment have less than 100 % peak transmission efficiency for p- and s- polarizations. It is important to note that the angular distribution shown in is much sharper than those found for fluorophores on uncoated glass, and sharper than those due to reflections above a mirror.
Angle-dependent intensities of (a) fluorescein and (b) rhodamine B containing PVA films on 40 nm thick platinum coated quartz slides.
We measured emission spectra of thin PVA films containing either fluorescein or rhodamine B on Pt films in both free-space and SPCE configuration to confirm the absence of scattered light. The emission spectra were recorded at the angle of maximum emission as shown in . The emission spectra on the SPCE side are spectrally equivalent to the free-space emission spectra for both fluorescein and rhodamine B samples (). We have also recorded the SPCE spectra through a polarizer as shown in . The SPCE was almost completely polarized in the horizontal direction, which is p
-polarized. The high polarization value demonstrates that SPCE signal is due to plasmons which, because of the wavevector matching conditions are created by the excited–state fluorophores. Furthermore, the SPCE signal is p
-polarized irrespective of the polarization of the incident light. The sharp angular distribution and p
-polarization are characteristic of SPCE [11
]. Our SPCE observations with reverse- Kretschmann (RK) configuration support that the surface plasmon coupling occurs in the excited state and that the SPCE is not directly created by the incident light. This suggests that the SPCE emission observed with thin Pt films is not due to direct transmission of fluorescence, but rather due to increased electromagnetic coupling efficiency between the excited state fluorophores and smooth Pt films.
Figure 3 Top Panel: Free-space and surface plasmon-coupled emission spectra of (a) fluorescein and (b) rhodamine-B in PVA films. Bottom Panel: Polarized and unpolarized surface plasmon-coupled emission spectra of (c) fluorescein and (d) rhodamine-B in PVA films. (more ...)
shows intensity decays for the free-space emission and the SPCE emission for thin PVA films of fluorescein and rhodamine B on thin platinum films. We carefully considered possible artifacts and the effects of sample geometry, while recording the intensity decays. The most important conclusion from these intensity decay measurements is that the lifetimes of SPCE and the free space emission are essentially the same.
Intensity-time decays of (a) fluorescein and (b) rhodamine containing PVA films in the free space (blue) and SPCE (black) configuration. IRF is also included in the plots.
In summary, we have examined fluorescein and rhodamine B in thin polymer films on 40 nm thick Pt films on quartz substrates. We observed efficient SPCE through thin Pt films. This suggests that the energy effectively coupled through the Pt film into the quartz substrate at a sharply defined angle. A significant portion of the total emission appeared as SPCE. The SPCE emission was p-polarized with different wavelengths appearing at different angles. The high polarization indicates that the emission is probably due to surface plasmons which radiate into the prism. The emission lifetimes of fluorophores were mostly unchanged in the free-space and SPCE modes.
The phenomenon of SPCE has now been reported for four metals Al, Ag, Au and Pt. These metals are biologically benign and if needed can be coated with silica for which there is a great deal of surface chemistry available for linking biomolecules. Additionally, these metals are easily used in nanofabrication methods such as focused ion beam (FIB) and e-beam nanolithography (EBL). We expect SPCE to become widely used in devices for DNA and protein assays, and in diagnostic systems.