Abstract
Samples of 0.01% and 0.3% Tm(3+)-doped β-NaYF4 show upconverted UV luminescence at 27 660 cm(-1) (361 nm) after blue excitation at 21 140 cm(-1) (473 nm). Contradictory upconversion mechanisms in the literature are reviewed and two of them are investigated in detail. Their agreement with emission and two-color excitation experiments is examined
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