Stefan R. Lüthi, Markus Pollnau, Hans U. Güdel and Markus P. Hehlen
Near-infrared to visible upconversion in Er³⁺-doped Cs₃Lu₂Cl₉, Cs₃Lu₂Br₉ and Cs₃Y₂I₉ excited at 1.54 mm
Phys. Rev. B 60, 162-178 (1999)                 

Abstract: A detailed study of upconversion processes in Cs₃Er₂X₉ (X = Cl, Br, I) crystals and in the diluted systems Cs₃Lu₂Cl₉:1% Er³⁺, Cs₃Lu₂Br₉:1% Er³⁺, and Cs₃Y₂I₉:1% 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 UV 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 ⁴I_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 (ETU) in all excitation steps. Of the 35 most likely ETU processes, eight are found to contribute significantly to the excitation mechanisms in the concentrated system. The excitation pathways leading to red luminescence from ⁴F_9/2 are also partly resolved.