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 colour tuning LEDs and lasers, light harvesting, sensing systems, and quantum computing. Despite its importance, fundamental questions still remain unanswered on the FRET rate dependency on the local density of optical states (LDOS). In this work, we investigate this directly, 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 5 times enhancement in the emission rate of the acceptor, resulted in an overall 14 times 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.

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Successful control of Förster resonance energy transfer (FRET) through the engineering of the local density of optical states (LDOS) will allow us to develop novel strategies to fully exploit this phenomenon in key enabling technologies. Here we present an experimental and theoretical study on the effect of the LDOS on the FRET rate and efficiency in plasmonic nanopatch antennas formed between a gold nanoparticle and an extended silver film. Our results reveal that plasmonic nanopatch antennas of similar levels of LDOS exhibit comparable levels of FRET rate and FRET efficiency, demonstrating that LDOS plays an important part in controlling both FRET rate and efficiency. Our findings contribute to the ongoing debate about the relation between the FRET process and the LDOS, as well as directly impacting the development of novel FRET based light harvesting and sensing devices.

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