Laser-relevant spectroscopy and upconversion mechanisms of Er³⁺ in Ba₂YCl₇ pumped at 800 nm

We present a comprehensive spectroscopic study of the possible room-temperature green laser transition 2H9/2 -> 4I13/2 in Ba2YCl7:3%Er3+. Because of the low phonon energies, 270 cm-1 in Ba2YCl7, the otherwise multiphonon-quenched 4I9/2 and 2H9/2 levels are metastable and can serve as intermediate pump and upper laser levels, respectively, for a green upconversion laser excited at 800 nm. Polarized spectra of ground-state and excited-state absorption at 800 nm and of emission at 560 nm are measured, and the corresponding absorption and emission cross sections are derived. Luminescence-decay measurements provide lifetime data. Despite the large number of metastable levels of Er3+ in a low-phonon host material, luminescence spectra and intensity-versus-power measurements reveal that the energy dissipation into levels other than those required for the operation of the laser transition (i.e., 4I15/2 -> 4I9/2 -> 2H9/2 -> 4I13/2) is small at low dopant concentrations. At higher concentrations, an energy-transfer upconversion process populates the lower laser level and counteracts inversion. The theoretical pump threshold of the proposed upconversion-laser transition under cw and pulsed excitation is derived.