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Suyver, J. F., Aebischer, A., Biner, D., Gerner, P., Grimm, J., Heer, S., … Güdel, H. U. (2005). Novel materials doped with trivalent lanthanides and transition metal ions showing near-infrared to visible photon upconversion. Optical Materials, 27(6), 1111–1130. doi:10.1016/j.optmat.2004.10.021 
10.1016/j.optmat.2004.10.021