(a) Sketch of double nanohole (DNH) structure to enhance single molecule fluorescence in the apex region. (b,c) Local intensity enhancement (linear scale) for a DNH of 25 nm gap and 190 nm diameter excited at 633 nm with a linear polarization perpendicular (b) and parallel (c) to the apex between the holes, taken in a plane 5 nm below the top metal surface. The inserts show the intensity enhancement along a vertical cut in the DNH center. All images share the same colorscale. (d) Scanning electron microscope image of the structure milled in 100 nm thick gold film using focused ion beam. (e) Experimental and (f) simulated transmission spectra for a DNH illuminated with normal incidence for two orthogonal linear polarizations along the apex (red) and perpendicular to the apex (blue).