Enhancement of single molecule fluorescence using conical micromirrors

Subwavelength metal apertures significantly enhance single molecule fluorescence signaling systems, but require efficient illumination and collection optics. On-chip micromirror structures offer a way to markedly improve the coupling efficiency between such subwavelength metal apertures and the external fluorescence illumination and collection optics, which in turn greatly simplifies several aspects of instrument design including optics, optomechanics, and thermal control. Modeling and experimental verification of the gains in illumination and collection efficiency for subwavelength metal apertures leads to a micromirror design that is both highly efficient yet also manufacturable. A combination of ray-based and finite-difference-time-domain models is used to optimize conical micromirrors colocated with subwavelength metal apertures for the case where the illumination light interacts strongly with the micromirror and the collection optics have modest numerical aperture (NA similar to 0.5). Experimental methods employing either freely diffusing or immobilized dye molecules are used to measure the illumination and collection efficiencies of fabricated micromirror prototypes. An overall fluorescence gain of similar to 100x, comprising a 20x improvement with flood illumination efficiency together with a 5x improvement in collection efficiency, are both predicted and experimentally verified.