A.A. Lyapin, S.V. Gushchin, A.S. Ermakov, S.V. Kuznetsov, P.A. Ryabochkina, V.Yu. Proydakova, V.V. Voronov, P.P. Fedorov, and M.V. Chernov. Mechanisms and absolute quantum yield of upconversion luminescence of fluoride phosphors / Chinese Optics Letters, Vol. 16, Issue 9, 091901 (2018) doi.org/10.3788/COL201816.091901