Controlling fluorescent proteins by manipulating the local density of photonic states

We present the first demonstration of control of the emission lifetime of a biological emitter by manipulating the local density of optical states (LDOS). LDOS control is achieved by positioning the emitters at defined distances from a metallic mirror. This results in a characteristic oscillation in the fluorescence decay rate. Since only the emitting species contribute to the emission lifetimes, the radiative and nonradiative decay rates derived from the lifetime changes characterize specifically the on- states of the emitter. We have thus experimentally determined the decay rates, and by extension the quantum efficiency and emission oscillator strength, of exclusively the emitting states of the widely used Enhanced Green Fluorescent Protein (EGFP). This approach is in contrast to other methods that average over emitting and dark states. The quantum efficiency of the on-states determined for EGFP is 72%. This value is higher than previously reported values determined by methods that average over on- and off-states, as is expected for this system with known dark states. The method presented is especially interesting for photophysically complex systems like fluorescent proteins, where a range of emitting and dark forms has been observed.