Photon upconversion could enable to harvest parts of the solar spectrum that cannot be absorbed in conventional silicon solar cells. However, typical upconversion materials suffer from a low absorption cross-section and a low brightness. Using the sun as energy source, signal enhancement strategies for efficient photon upconversion are required, e.g. by the utilization of metasurfaces providing strongly enhanced electrical near-fields. Here, we placed β-NaYF4:Er3+ nanoparticles on large-area silicon metasurfaces designed to convert near-infrared (1550 nm) excitation light to visible luminescence. We observed a more than 2400-fold enhanced photon upconversion luminescence compared to a planar substrate. Optical simulations allowed attributing this result to the coupling of the excitation source with metasurface resonances at appropriate incident angles. Aiming at broadband applications such as solar energy conversion we also introduced a multi-layer metasurface design: Comparing multi-layer to conventional single-layer structures revealed that the resonances associated with enhanced near-fields are split into multiple different modes and are spectrally broadened. The findings not only permit the significant reduction of the excitation power densities required for photon upconversion of 1550 nm light, but also introduce a new approach for efficient upconversion under broadband excitation conditions. This can open new perspectives for applications of such materials in the third biological excitation window, telecommunication, and photovoltaics.
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