Markus Wermuth, Toni Riedener and Hans U. Güdel
Spectroscopy and upconversion mechanisms of CsCdBr₃:Dy³⁺
Phys. Rev. B 57, 4369-4376 (1998)                 

Abstract: The upconversion luminescence of a Dy³⁺-doped system is presented and analyzed. CsCdBr₃:x% Dy³⁺ (x = 0.2, 1, 5) was synthesized and grown as crystals using the Bridgman technique. Dy³⁺ ions preferentially enter this host as charge-compensated dimers. Due to the low-phonon energy the efficiency of multiphonon-relaxation processes is significantly reduced in this host compared to oxides and fluorides. Yellow-green upconversion luminescence originating from ⁴F_9/2 can be induced upon excitation into ⁶F_5/2 or ⁶F_3/2 in the near infrared (NIR). Depending on the excitation wavelength, upconversion occurs by an energy-transfer or excited-state absorption mechanism. The two are distinguished by their temporal behavior after an excitation pulse. Analysis of the upconversion-luminescence transient of CsCdBr₃:0.2% Dy³⁺ at 10 K leads to a rate constant W_t = 165 s^–1 for the energy-transfer step. This is very small, and thus upconversion based on one excitation wavelength is inefficient. In addition, the intermediate NIR level ⁶F_5/2 is significantly depopulated by multiphonon relaxation at room temperature.