Fluorescence resonance energy transfer (FRET) is an important photophysical mechanism which finds many applications in biophotonics, particularly in biological sensing and imaging. It is well known, there are two major factors that determine the efficiency of FRET. One is the distance between the donor and the acceptor, and the other is the overlap of the donor's emission and the acceptor's absorption spectra. However, while the distance-modulation of FRET is very popular, the spectral-overlap-m