Sara García-Revilla, Pascal Gerner, Hans U. Güdel and Rafael Valiente
Yb³⁺-sensitized visible Ni²⁺ photon upconversion in codoped CsCdBr₃ and CsMgBr₃
Phys. Rev. B 72, 125111/1-9 (2005)                 

Abstract: Near-infrared excitation of Yb^3+ 2F_7/2 ²F_5/2 at 10600 cm^–1 in the linear chain CsMBr₃:Ni²⁺,Yb³⁺ (M=Cd,Mg) structure leads to green luminescence from Yb³⁺ pairs and red Ni²⁺ upconversion luminescence at cryogenic temperatures. The broad red upconversion luminescence is assigned to the ¹T_2g ³A_2g transition of Ni²⁺. The upconversion excitation spectrum indicates that the active mechanism in these systems involves both Yb³⁺ and Ni²⁺ ions. Lifetime and power dependence data allow one to identify a ground-state-absorption (GSA) energy-transfer upconversion (ETU) as the underlying upconversion mechanism for the Yb³⁺-Ni²⁺ system in doubly doped CsCdBr₃ and CsMgBr₃. A less efficient single-ion GSA excited-state-absorption (ESA) process within Ni²⁺ is also observed in the title compounds. However, whereas the GSA/ESA process requires an excitation energy above 11200 cm^–1, it is possible to sensitize the Ni²⁺ upconversion via Yb³⁺ excitation at energies around 10200 cm^–1 and around 10600 cm^–1 in CsCdBr₃ and CsMgBr₃. It is shown that only those Ni²⁺ ions neighboring to Yb³⁺ ions are relevant for the upconversion induced by Yb³⁺ excitation.