Abstract
In this work we report infrared to visible upconversion luminescence in the low phonon-energy host material ${\mathrm{KPb}}_{2}{\mathrm{Cl}}_{5}$ doped with ${\mathrm{Er}}^{3+}$ 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 $(\ensuremath{\approx}800\mathrm{nm}),$ 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.
Highlights
The interest in upconversion emission in rareearthREdoped materials has been increased because of the search for all solid-state compact laser devices operating in the visible region and the availability of powerful nearinfrared laser diodes
In this work we report infrared to visible upconversion luminescence in the low phonon-energy host material KPb2Cl5 doped with Er3ϩ ions under 4I9/2 excitation
The 4I9/2 state acts as a sensitizer for both Er3ϩ ions and an excitation spectrum similar to the OP absorption spectrum is expected for the ETU process
Summary
The interest in upconversion emission in rareearthREdoped materials has been increased because of the search for all solid-state compact laser devices operating in the visible region and the availability of powerful nearinfrared laser diodes. The advantage of sulfide- and chloride-based hosts over the most extensively studied fluoride compounds is the lower phonon energy that leads to a significant reduction of the multiphonon relaxation rates. This allows an increased lifetime of some excited levels that can relax radiatively or can store energy for further upconversion, cross-relaxation, or energy-transfer processes. These materials usually present poor mechanical properties, moisture sensitivity, and are difficult to synthesize. Efficient infrared to visible upconversion in this crystal when doped with Pr3ϩ ions and co-doped with Yb3ϩ ions has been recently reported by our group.
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