R. Burlot-Loison, M. Pollnau, K. Krämer, P. Egger, J. Hulliger and H. U. Güdel
Laser-relevant spectroscopy and upconversion mechanisms of Er³⁺ in Ba₂YCl₇ pumped at 800 nm
J. Opt. Soc. Am. B 17, 2055-2067 (2000)                 

Abstract: We present a comprehensive spectroscopic study of the possible room-temperature green laser transition ²H_9/2 ⁴I_13/2 in Ba₂YCl₇:3%Er³⁺. Because of the low phonon energies, 270 cm^–1 in Ba₂YCl₇, the otherwise multiphonon-quenched ⁴I_9/2 and ²H_9/2 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., ⁴I_15/2 ⁴I_9/2 ²H_9/2 ⁴I_13/2) 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.