Motivated by the technologically important spectral conversion of sunlight for efficient photocatalysis, we present a detailed analysis of the multi-color excitation of LiYF 4 :Yb 3+ ,Tm 3+ nanocrystals leading to UV emission through upconversion. We demonstrate in particular that the combination of blue and IR light for generating upconversion UV emission is a linear mechanism that occurs at rather low density of excitation (a few mW/cm 2 ). This up-conversion efficiency is fully compatible

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... h 1 the broad band excitation from the sun, with comparable density similar to daylight excitation. It demonstrates that an appropriate design combining photocatalyst and dedicated up-converter leads to a drastic improvement of the photocatalytic effect.