Is There a Proper Figure of Merit for a Plasmonic Structure Involved in Metal-Enhanced Fluorescence?

A suitability of a nanostructure for a metal-enhanced fluorescence (MEF) is usually assessed from the value of the maximal fluorescence enhancement factor, F, which it can generate. However, F is an ambiguous quantity which may dramatically depend on the intrinsic quantum yield, q(0), of the emitter in a free space. Here, we suggest F = F-q0=1 taken in the limiting case of q(0) = 1 as a proper figure of merit for a plasmonic structure involved in MEF. Any other realistic F in the MEF regime can be recovered from F-q0=1 by a simple 1/q(0) scaling.