Förster Resonance Energy Transfer and the Local Optical Density of States in Plasmonic Nanogaps.

Förster resonance energy transfer (FRET) is a fundamental phenomenon in photosynthesis and is of increasing importance for the development and enhancement of a wide range of optoelectronic devices, including color-tuning LEDs and lasers, light harvesting, sensing systems, and quantum computing. Despite its importance, fundamental questions remain unanswered on the FRET rate dependency on the local density of optical states (LDOS). In this work, we investigate this directly, both theoretically and experimentally, using 30 nm plasmonic nanogaps formed between a silver nanoparticle and an extended silver film, in which the LDOS can be controlled using the size of the silver nanoparticle. Experimentally, uranin-rhodamine 6G donor-acceptor pairs coupled to such nanogaps yielded FRET rate enhancements of 3.6 times. This, combined with a 5-fold enhancement in the emission rate of the acceptor, resulted in an overall 14-fold enhancement in the acceptor's emission intensity. By tuning the nanoparticle size, we also show that the FRET rate in those systems is linearly dependent on the LDOS, a result which is directly supported by our finite difference time domain (FDTD) calculations. Our results provide a simple but powerful method to control FRET rate via a direct LDOS modification.