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Upconversion fluorescence from Lanthanide-doped nanocrystals has attracted widespread interests because of its greatly potential applications in various fields, such as photonic crystal lasers, material science, biological therapy, and so on. However, the relatively low quantum yield (typically < 5%) is the major limitation for upconversion nanocrystals. Meanwhile, in addition to the chemical methods, plasmonic structures have been adopted as another strategy to improve the radiation efficiency and control the relaxation process of the upcovnersion nanocrystals. We designed the anti-symmetric split ring resonators with various periods and the fishnet structures. The surface plasmon resonance peaks of the structure shift as the periods varies. For example, in a multi-layered plamsonic metasurface with the period of 250nm, both the electric and magnetic modes could be generated simultaneously when excited by the incident light with proper polarization. This plasmonic structure provides two different channels for the enhancement of upconversion fluorescence. The resonance peak of 650nm is magnetic resonance mode, while the peak of 980nm is electric resonance mode. The resonance peak of 980nm coincides with the absorption band of the Lanthanide-dopoed nanocrystal, and the peak of 650nm matches with its emission band. We found that the upconversion fluorescence intensity could be enhanced more than 10 times when the electric resonance frequency of the metasurface matches with the absorption band of the upconversion nanocrystals, while the magnetic mode overlaps with its emission band. This is due to the local density of optical states was significantly enhanced by the plasmonic metasurface. The detailed results and mechanism will be discussed.
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