Photon Avalanche in Cs₂ZrBr₆:Os⁴⁺

Abstract

The 15 K visible (VIS) upconversion-luminescence of Cs2ZrBr6:0.2% Os4+ upon excitation in the near-infrared (NIR) is presented and analyzed. Crystals of Cs2ZrBr6:0.2% Os4+ were synthesized and grown by the Bridgman technique. At 15 K VIS luminescence around 16000 cm-1 is induced by NIR excitation around 11400 and 13400 cm-1. Due to the low maximum vibrational energy of 220 cm-1, four excited states participate in the upconversion processes. A photon avalanche and a ground-state absorption/excited-state absorption sequence are identified as upconversion mechanisms by high-resolution upconversion-excitation spectra and by power-dependent and time-resolved measurements. The lifetimes of the excited states Γ4 (3 T1g), Γ5 (3 T1g), Γ5 (1 T2g), and Γ1 (1 A1g) at 15 K are 45 ms, 1.6 ms, 261 μs, and 8 μs, respectively. The dynamics of the upconversion are simulated with a rate-equation model.