Nanocrystal-Sensitized Infrared-to-Visible Upconversion in a Microcavity under Subsolar Flux.

Infrared-to-visible photon upconversion could benefit applications such as photovoltaics, infrared sensing, and bioimaging. Solid-state upconversion based on triplet exciton annihilation sensitized by nanocrystals is one of the most promising approaches, albeit limited by relatively weak optical absorption. Her; we integrate the upconverting layers into a Fabry-Perot microcavity with quality factor Q= 75. At the resonant wavelength A = 980 nm, absorption increases 74-fold and we observe a 227-fold increase in the intensity of upconverted emission. The threshold excitation intensity is reduced by 2 orders of magnitude to a subsolar flux of 13 mW/cm(2). We measure an external quantum efficiency of 0.06 +/- 0.01% and a 2.2-fold increase in the generation yield of upconverted photons. Our work highlights the potential of triplet-triplet annihilation-based upconversion in low-intensity sensing applications and demonstrates the importance of photonic designs in addition to materials engineering to improve the efficiency of solid-state upconversion.