Förster Resonance Energy Transfer inside Hyperbolic Metamaterials

The ability to control Förster resonance energy transfer (FRET) between emitters via the design of nanostructured materials with appropriate electromagnetic properties is important in the development of fast and enhanced sources of illumination, high-efficiency photovoltaic devices, and biomedical applications, such as nanorulers. While the engineering of the local density of states allows an efficient control over the spontaneous emission rate, its influence on the FRET process has been an ongoing debate and has led to disparate experimental and theoretical results. Here, we experimentally demonstrate an increase in the FRET rate for donor-acceptor (D-A) pairs separated by fixed distances (3.4, 6.8, and 10.2 nm) located inside a hyperbolic metamaterial comprised of an array of gold nanorods. While the modification of the local density of states surrounding the D-A pairs strongly influences the FRET rate, leading to a 13-fold increase inside the metamaterial, the FRET efficiency is shown to remain mostly unaffected. For comparison, we also study the modification of the energy transfer rate and efficiency for the D-A pairs placed on top of a gold film, on top of a nanorod-based metamaterial, and inside a nanorod-based metamaterial coated with polymer in order to prevent quenching. The free-space emission intensity of the acceptor, in the presence of FRET, was also investigated, leading to an 18-fold increase of the emission intensity for the D-A pairs inside the metamaterial. The designed geometry shows great potential in the development of FRET-based applications such as biomedical imaging, organic solar cells, and light-emitting sources.