Enhancing Efficiency and Directionality of Photon Upconversion Via Bound States in the Continuum

Upconversion (UC) of infrared light into visible light has wide potential applications, such as bioimaging, 3D displays, and anti-counterfeiting. However, current UC efficiency is low, whether by organic-based triplet-triplet annihilation (TTA) or lanthanide-doped upconversion nanoparticles (UCNPs). Here, we utilize dielectric metasurfaces with high quality factors, specifically arrays of titanium dioxide (TiO₂) cylinders that support bound states in the continuum (BICs), to enhance the UC efficiency. For instance, a bilayer film of quantum dots and organics for TTA-UC is deposited on a TiO₂ array, which gives a quadrupole-based symmetry-protected BIC at the pump wavelength of 976 nm. Efficient coupling leads to nearly two orders of magnitude of increase in quantum dot absorption and hence the UC efficiency. In contrast, a layer of UCNPs is deposited on a TiO₂ array, which results in a Friedrich-Wintgen BIC at the emission wavelength of 654 nm. A highly directional beam is produced, albeit with a limited increase in the UC efficiency. Our work points to a design principle that maximizes absorption for efficiency enhancement and modifies emission for directionality control.