Toni Riedener, Philipp Egger, Jürg Hulliger and Hans U. Güdel
Upconversion mechanisms in Er³⁺-doped Ba₂YCl₇
Phys. Rev. B 56, 1800-1808 (1997)                 

Abstract: Efficient near infrared to visible upconversion luminescence is reported in the low phonon-energy host material Ba₂YCl₇:x% Er³⁺ (x=1, 10, 100). The upconversion mechanisms upon ⁴I_9/2 excitation ( 800 nm) are investigated by means of excitation and time-resolved luminescence spectroscopy. ²H_9/2 is found to be populated by energy transfer upconversion and excited-state absorption, depending on the excitation energy. Characteristic excited-state absorption peaks in the excitation spectra are used as a fingerprint to determine the population mechanisms for the other states with high luminescence intensities: ⁴S_3/2/²H_11/2, ⁴F_9/2, and ⁴G_11/2. The assignment is confirmed by temperature, concentration, power, and time-dependent measurements. The relative intensities are accounted for by a Judd-Ofelt analysis.