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Under 980 nm excitation, high multi-photon upconversion (UC) emission from the 2H11/2/4S3/2 (green) and 4F9/2 (red) levels of Er3+ ions were observed from Er3+ singly doped BiOCl microcrystals. These high-energy excited states were populated by a three to ten photon UC process conditionally, which depended on the pump power density and the Er3+ ion doping concentration, characterizing as a hetero-looping enhanced energy transfer avalanche UC process. UC emission lifetime and Raman analysis suggest that the unusual UC phenomena are initiated by the new and intense phonon vibration modes of BiOCl lattices due to Er3+ ions doping.
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Li et al. (2013) studied this question.
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