Near-Infrared to Visible Upconversion in Er3+ Doped Cs3Lu2Cl9, Cs3Lu2Br9, and Cs3Y2I9 Excited at 1.54 MM

A detailed study of upconversion processes in Cs₃Er₂X₉ (X = Cl, Br, I) crystals and in the diluted systems Cs₃Lu₂Cl₉ : 1% Er³⁺, and Cs₃Y₂l₉ : 1% Er³⁺ is presented. Efficient two-, three-, and four-step upconversion excitation along the sequence ⁴I_15/2 → ⁴I_13/2 → ⁴I_9/2 → ⁴S_3/2 → ²H_9/2, leading to luminescence throughout the visible and near-ultraviolet is demonstrated using a 1.54 mm excitation wavelength. This stepwise excitation is possible due to the low phonon energies and, consequently, the significantly longer lifetime of the ⁴l_9/2 intermediate state in these systems relative to oxides and fluorides. The absorption and upconversion luminescence intensities increase along the isostructural series X= Cl, BR, I as a result of the decreasing energy of the electric-dipole allowed 4f-5d transitions and, thus, their increasing influence on the parity forbidden 4f-4f transitions. The excitation mechanisms in the chloride systems are investigated by time-resolved spectroscopy and the respective dynamics is studied by a rate-equation model. In the diluted sample ⁴I_9/2 → ⁴S_3/2 excited-state absorption plays a major role and occurs within 3 cm^-1 of the ground-state absorption, whereas the dynamics in the concentrated system is dominated by energy-transfer upconversion in all excitation steps. Of the 35 most likely energy-transfer upconversion processes, eight are found to contribute significantly to the excitation mechanisms in the concentrated system.