Direct imaging of the energy transfer enhancement between two dipoles in a photonic cavity

Photonic cavities are gathering a large interest to enhance the energy transfer between two dipoles, with far-reaching consequences for applications in photovoltaics, lighting sources and molecular biosensing. However, experimental difficulties in controlling the dipoles' positions, orientations and spectra have limited the earlier work in the visible part of the spectrum, and have led to inconsistent results. Here, we directly map the energy transfer of microwaves between two dipoles inside a resonant half-wavelength cavity with ultrahigh control in space and frequency. Our approach extends F\"orster resonance energy transfer (FRET) theory to microwave frequencies, and bridges the gap between the descriptions of FRET using quantum electrodynamics and microwave engineering. Beyond the conceptual interest, we show how this approach can be used to optimize the design of photonic cavities to enhance dipole-dipole interactions and FRET.