In this work we report infrared to visible upconversion luminescence in the low phonon-energy host material KPb 2 Cl 5 doped with Er 3ϩ ions under 4 I 9/2 excitation. The upconversion mechanisms are investigated by using steady-state and time-resolved laser spectroscopy. When the excitation wavelength is resonant with the 4 I 9/2 level (Ϸ800 nm), the upconverted emission originates from the levels 2 H 9/2 and 4 S 3/2 . These upconverted emissions occur via energy-transfer upconversion processes. However, under nonresonant excitation at lower energies than 4 I 9/2 , the main emission results from the level 2 H 9/2 . This latter upconverted emission can be attributed to excited-state absorption of the pump radiation. The proposed upconversion mechanisms responsible for the different emissions from the levels 2 H 9/2 and 4 S 3/2 are supported by both the time evolution of the upconversion luminescence after pulsed infrared excitation and the upconversion luminescence excitation spectra. Rate equation analysis has been used to identify and characterize the energy-transfer processes responsible for the observed fluorescence behavior.

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We report internal laser cooling in Yb(3+) -doped KPb(2)Cl(5) . From the quantum efficiency values measured in the heating and cooling regions by use of the photothermal deflection technique, we have obtained a room-temperature cooling efficiency of 0.2% in a sample doped with ~5x10(19)ions/cm(3) . Excitation spectra obtained under high irradiation fluences show an excess of fluorescence with regard to those obtained at low fluences, which agrees with the prediction of a model based on photon-ion-phonon interaction.

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