Significant progress has been made in synthesizing and characterizing ultra-pure, rare-earth-doped ZIBLAN (ZrF4-InF3-
BaF2-LaF3-AlF3-NaF) glass capable of laser refrigeration. Yb3+-doped ZIBLAN glass was produced from fluoride
precursors which were individually purified by solvent extraction and subsequently treated with hydrofluoric gas at
elevated temperatures to remove oxygen impurities. We have developed two-band differential luminescence
thermometry (TBDLT) as a new non-invasive, spectroscopic technique to evaluate the intrinsic quality of Yb3+ doped
laser-cooling samples. TBDLT measures changes in the local temperature upon laser excitation via the small changes in
the 2F5/2→2F7/2 fluorescence spectrum. Two commercial band pass filters in combination with a balanced dual InGaAs
photodetector are used to select and integrate regions of interest in the fluorescence spectrum with sub-millisecond
resolution. The TBDLT technique successfully finds the zero-crossing temperature (ZCT), which is the minimum
temperature to which a Yb3+ doped sample can cool, independent of surface heating. ZCT is a useful measure for the
presence of impurities and the overall quality of the laser-cooling material. Favorable laser cooling results were obtained
for several 1% Yb3+-doped glasses with varying degrees of purity.
The fluorozirconate glass ZBLAN:1%Yb3+ was synthesized, for the first time, from fluoride precursors that were
individually purified by solvent extraction and hydrofluoric (HF) gas treatment. The synthesis used aqueous solutions of
high-purity commercial precursors that were subjected to ultra-filtration followed by solvent extraction using ammonium
pyrrolidine dithiocarbamate (APDC) and methyl-isobutyl-ketone (MIBK). The purified metal fluorides were precipitated
and treated in hot HF gas to remove water, hydroxyl (OH-), and oxide impurities. ZBLAN:1%Yb3+ was fabricated from
these precursors by melting under inert atmosphere, yielding glasses with excellent mechanical properties and having a
clear, bubble-free, and crystallite-free matrix. The effect of adding 0.5 mol% of In3+ as an oxidizer to suppress the
reduction of Zr4+ and the accompanying formation of black precipitates was studied. We found evidence for an oxidizer
concentration threshold of ~0.8 mol%. Glasses made from purified fluorides formed black precipitates even with the
addition of 0.5 mol% In3+, while glasses made from commercial fluorides did not. In the latter, additional oxidizers were
likely present in the form of transition-metal impurities. An In3+ oxidizer concentration of >0.8 mol% is expected to
eliminate the black precipitates in purified glasses and to yield ZBLAN:Yb3+ glass for efficient laser cooling.
We have developed a non-contact spectroscopic technique to measure the temperature change of Thulium-doped glasses and crystals with high precision. The approach makes use of temperature-dependent broadening of various peaks in the luminescence spectra. A weak, cw probe laser excites the sample in vacuum. The luminescence spectrum is collected by a fiber and routed to a high-resolution spectrometer. We have demonstrated temperature resolution of 62 mK in Tm:BaY2F8.
We present an overview of laser cooling of solids. In this
all-solid-state approach to refrigeration, heat is removed radiatively when an engineered material is exposed to high power laser light. We report a record amount of net cooling (88 K below ambient) that has been achieved with a sample made from doped fluoride glass. Issues involved in the design of a practical laser cooler are presented. The possibility of laser cooling of semiconductor sensors is discussed.