Nanoaperture-enhanced fluorescence: Towards higher detection rates with plasmonic metals

A bare nanometric aperture milled in a metallic film forms a simple nanophotonic device that can strongly enhance the optical properties of nearby emitters such as fluorescent molecules. In this paper, we experimentally and numerically compare the properties of circular apertures milled in gold and aluminum, and discuss the influence of a noble metal holding plasmonic resonances in the visible range such as gold. We report that nanometric apertures milled in gold exhibit significantly higher fluorescence enhancement factors than apertures in aluminum. We relate this effect to a larger enhancement of the excitation intensity and radiative rate for an aperture milled in gold. A spectrally resolved analysis of the fluorescence emission from apertures is also presented. Comparison with numerical simulations shows that the enhancement factor is maximum when the photonic density of modes is maximum. Altogether, these results provide crucial knowledge for the design of nanoapertures towards high-efficiency single-molecule analysis.