Emission enhancement nanoaperture-enhanced fluorescence

We therefore assume that the major effect of the array is in increased collection of light within the apertures. In the simulation results of this section, we only consider briefly the excitation component to enhanced fluorescence emission. As already established by experiments, the light intensity within periodic arrangements of nanoapertures increases in association with EOT and is a result of aperture coupling effects via surface waves under external excitation. This coupling therefore modulates the response obtained for an isolated aperture. Simulations were performed in the same manner as described before, except for the fact that periodic boundary conditions were used along the sides of the simulaticHi space boundary. 

In summary, we have tried to overview the state of the field of nanoaperture enhanced fluorescence. While much is known currently about the photophysics of an isolated aperture, it is clear that much more work needs to be performed in order to understand, and maximize, fluorescence enhancement effects from arrangements of apertures and structured apertures. In particular, the relative contributicms of radiative and non-radiative processes have (Hily beoi studied in a couple of experiments, and limited computaticHial work has been performed. In addition, the role of emission directicHiality is not clear at present and requires further study. 

Figure 17,8 Fluorescence enhancement at saturation of the absorption / emission cycle, for A647 molecules in single nanoapertures milled in gold. This factor represents the product of the gain in collection efficiency by the enhancement in radiative rate Okrad- This figure highlights the contributicsi of the enhanced emission independently of the altered excitation intensity.

Dipole emission is a rather more complicated process to simulate than excitation, so some simplifications will be made here. The calculations presented here follow that of reference (46), in which the increase in radiative output of a dipole lying within a nanoaperture, as measured in a plane just below the interface with the substrate, is determined. The enhancement is calculated by taking the ratio of the power flow through this plane divided by the power flow through this same plane, but in the absence of the metal, for all three dipole orientations. This calculated enhancement corresponds approximately to the measured fluorescence... 

Again, the emission pattern from a glass substrate is also shown for reference. Figure 17.18 shows the fluorescence output as a function of incidence angle for the passivated sample. At the surface-plasmon incidence angle, the total fluorescence enhancement compared to the reference is 12 (normalized to the 3.1% fill-fraction of the bottom surface of the nanoapertures), which is comparable to the enhancement obtained under full interior surface coverage (Figure 17.14). Therefore, the fluorescence enhancement (per unit area) is comparable for fluorophores on the bottom as for fluorophores on the sidewalls with backside detection, as before with individual apertures. 

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