Efficient near infrared to visible upconversion luminescence is reported in the low phonon-energy host material ${\mathrm{Ba}}_2{\mathrm{YCl}}_7:x\%$ ${\mathrm{Er}}^{3+}$ ($x=1,$ 10, 100). The upconversion mechanisms upon ${}^4{I}_{9/2}$ excitation $\left(\approx 800\mathrm{nm}\right)$ are investigated by means of excitation and time-resolved luminescence spectroscopy. ${}^2{H}_{9/2}$ 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: ${}^4{S}_{3/2}{/}^2{H}_{11/2},$ ${}^4{F}_{9/2},$ and ${}^4{G}_{11/2}.$ The assignment is confirmed by temperature, concentration, power, and time-dependent measurements. The relative intensities are accounted for by a Judd-Ofelt analysis.

• Received 31 January 1997

DOI:https://doi.org/10.1103/PhysRevB.56.1800