Förster resonance energy transfer in inhomogeneous and absorptive environment

We present an analytical model for Förster resonance energy transfer (FRET) between a donor and an acceptor placed in an inhomogeneous and absorptive environment characterized by a complex dielectric function, e.g., near a metal-dielectric structure. By extending the standard approach to FRET to include energy transfer (ET) channel to the environment, we show that, in the absence of plasmonic enhancement effects, the Förster radius, which defines the characteristic distance for efficient FRET, is reduced due to a competing ET process. We demonstrate that the reduction in the Förster radius can dramatically affect fluorescence from large ensemble of molecules whose emission kinetics is dominated by FRET-induced concentration quenching. In particular, we perform numerical calculations for dye-doped polymer films deposited on top of a metallic substrate to find that, at high dye concentrations, the emission kinetics slows down considerably as compared to the same films on a glass substrate, in sharp contrast to acceleration of single-molecule fluorescence near the metal. Furthermore, the effective fluorescence decay rate exhibits a non-monotonic behavior with varying film thickness, consistent with the experiment, indicating a non-trivial interplay between the metal quenching and concentration quenching mechanisms.