Pascal Gerner, Oliver S. Wenger, Rafael Valiente and Hans U. Güdel
Green and Red Light Emission by Upconversion from the near-IR in Yb³⁺ Doped CsMnBr₃
Inorg. Chem. 40, 4534-4542 (2001)                 

Abstract: Direct near-IR excitation of Yb^{3+ 2}F_7/2 ²F_5/2 levels at 10126, 10138, and 10596cm^–1 in CsMnBr3:0.5%Yb³⁺ leads to three types of luminescence at cryogenic temperatures: near-IR Yb³⁺ emission and green and red upconverted luminescence. The green luminescence around 20000cm^–1 is identified as cooperative Yb³⁺ pair upconversion. The broad red upconversion luminescence band centered at 14700cm^–1 is ascribed to the ⁴T_1g ⁶A_1g transition of Mn²⁺. Pulsed measurements indicate a sequence of ground-state absorption and excited-state absorption steps for the red upconversion process. One- and two-color excitation experiments support this, and we conclude that the red upconversion occurs by an exchange mechanism involving Yb³⁺ and Mn²⁺. The Yb^{3+ 2}F_5/2 ²F_7/2 near-IR emission around 10000cm^–1 is also observed after Mn²⁺ excitation at 21838cm^–1. This is indicative of a Mn^{2+ 4}T_1g Yb^{3+ 2}F_5/2 relaxation process, which is a potential loss process for upconversion efficiency.