Lanthanide-doped nanocrystals have been actively pursued as anti-Stokes emitters in various contexts, including bioimaging, photovoltaics, catalysis, displays, anticounterfeiting, sensing and lasers. The success of these applications crucially relies on their high brightness under low excitation. To enhance their upconversion luminescence, researchers have improved the internal material properties of nanocrystals (their composition, doping, and crystal and surface structures) and, in parallel, engineered external optical responses using plasmonic couplings. However, despite impressive progress, upconversion brightness still falls short of what is required for applications, and a systematic understanding of plasmon-enhanced upconversion remains elusive. Here we report an important conceptual advance in understanding and demonstrate unprecedentedly bright upconversion of single nanocrystals via coupling to a single plasmonic nanocavity mode. We present in situ-controlled single-nanocrystal-level studies with unified internal and external treatment, and unambiguously experimentally demonstrate the phenomenon of plasmonic enhancement saturation. We show that the saturation is doping-dependent, and we report a 2.3 x 10(5)-fold enhancement of upconversion luminescence. More importantly, we outline a new strategy to devise ultrabright upconversion nanomaterials and demonstrate that single sub-30-nm nanocrystals can provide up to 560 detected photons per second at an ultralow excitation intensity of 0.45 W cm(-2). These findings help to establish the link between the optical physics and material science in lanthanide-doped nanocrystals and facilitate the engineering of optimal upconversion nanomaterials for various applications.
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