(a) Fluorescence brightness per molecule versus the excitation power for Alexa Fluor 647 with 200 mM methyl viologen (quantum yield ~8%). The data for the double nanohole with perpendicular orientation respective to the apex (blue) and the reference confocal data (green) are multiplied respectively by 2× and 10×. (b) Fluorescence enhancement factors with excitation polarization parallel (red) and perpendicular (blue) respective to the apex. Different concentrations of chemical quencher are used, corresponding to different values of quantum yield in solution: from left to right the data points correspond to methyl viologen concentrations of 200 mM, 80 mM and 0. For (b), the excitation power is 10 μW.