Extended nonradiative dipole-dipole energy transfer and spatiotemporal coherence enabled by bound states in the continuum.

Breaking the fundamental distance limit of dipole-dipole interactions is key to enabling scalable quantum and photonic technologies. Conventional nonradiative energy transfer [Förster resonance energy transfer (FRET)] is intrinsically confined to deep subwavelength distances, limiting its scalability. Here, we demonstrate millimeter-scale, coherent dipole-dipole energy transfer enabled by nonradiative bound states in the continuum (BICs) in a gold metasurface. Using a custom terahertz near-field time-domain microscope with dipolar emitter-detector probes, we observe strongly anisotropic energy transfer enhanced along one axis, where BIC modes establish long-range spatiotemporal coherence, and suppressed along the orthogonal axis. By systematically reducing the array size, we reveal that energy transfer efficiency and spatiotemporal coherence are maximized near the metasurface's center, where boundary-induced scattering is minimized. These findings establish BICs and quasi-BICs as robust, nonradiative channels for extended, directional, and coherence-preserving dipolar interactions, providing a versatile platform for quantum communication, integrated nanophotonics, and biosensing technologies.