Philipp Müller, Markus Wermuth and Hans U. Güdel
Mechanisms of near-infrared to visible upconversion in CsCdBr₃:Ho³⁺
Chem. Phys. Lett. 290, 105-111 (1998)                 

Abstract: Crystals of CsCdBr₃ doped with x% Ho³⁺ (x=0.35, 2.25) were synthesized and studied by high-resolution absorption, upconversion luminescence and excitation spectroscopy in the temperature range 295 - 4 K. The time evolution of the upconversion luminescence, within 1.5 ms of a pulsed excitation, was recorded. Two NIR to VIS upconversion mechanisms were identified. Below 100 K upconversion occurs by energy transfer within Ho³⁺ pairs. Above 100 K an excited-state absorption is dominant. It occurs whenever the energies of the ground- and excited-state absorption steps coincide. At room temperature this latter process overall is 5 times more efficient than the energy-transfer process.