Synthesis and evaluation of ultra-pure rare-earth-coped glass for laser refrigeration Page: 2 of 10
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Synthesis and evaluation of ultra-pure, rare-earth doped glass for laser
refrigeration
Wendy M. Pattersona* Markus P. Hehlen, Mansoor Sheik-Bahaea, Richard I. Epstein
aDept. of Physics and Astronomy, University of New Mexico, Albuquerque, NM 87131, USA;
bLos Alamos National Laboratory, Mailstop J565, Los Alamos, NM 87545, USA
ABSTRACT
Significant progress has been made in synthesizing and characterizing ultra-pure, rare-earth doped ZIBLAN (ZrF4-InF3-
BaF2-LaF-AlF3-NaF) glass capable of laser refrigeration. The glass was produced from fluorides which were
individually purified and subsequently treated with hydrofluoric gas at elevated temperatures to remove impurities
before glass formation. Several Yb3 +-doped samples were studied with varying degrees of purity and composition with
successive iterations producing an improved material. We have developed a non-invasive, spectroscopic technique, two
band differential luminescence thermometry (TBDLT), to evaluate the intrinsic quality of the ytterbium doped ZIBLAN
used for laser cooling experiments. TBDLT measures local temperature changes within an illuminated volume resulting
solely from changes in the relative thermal population of the excited state levels. This TBDLT technique utilizes two
commercially available band pass filters to select and integrate the "difference regions" of interest in the luminescence
spectra. The goal is to determine the minimum temperature to which the ytterbium sample can cool on the local scale,
un-phased by surface heating. This temperature where heating and cooling are exactly balanced is the zero crossing
temperature (ZCT) and can be used as a measure for the presence of impurities and the overall quality of the laser
cooling material. Overall, favorable results were obtained from 1% Yb3-doped glass, indicating our glasses are desirable
for laser refrigeration.
Keywords: optical refrigeration, laser cooling, ZBLAN, fluoride glass, rare-earth, fluoride purification, solvent
extraction, luminescence thermometry.
* wendv5 cunm.edu; phone 1 505 277-3301; fax 1 505 277-1013
1. INTRODUCTION
Our goal is to provide effective and relatively expedient feedback for our glass fabrication facilities, in order to
determine whether our purification techniques have been successful in increasing the laser induced cooling capabilities
of the material. An excellent introduction to laser refrigeration has been covered elsewhere [ref].
In our facilities, we synthesize rare-earth doped fluorozirconate glasses beginning with high purity starting materials to
produce the fluorides constituting the glass. These glass components must be processed in a clean room environment and
further purified in order to have the possibility of ever reaching laser cooling at cryogenic temps. This further
purification is necessary as we will need to reduce the impurities in even the current record holding ZBLAN glass by a
factor of 20-30 times to reach the target < 100 ppb impurity levels needed [ref]. However, even ZBLAN of this impurity
is no longer commercially available. We therefore must synthesize our own ZBLAN. In our context, an impurity is any
species in the material that lowers the external quantum yield of the rare-earth dopant. The primary quenching
mechanism is by non-radiative energy transfer from the rare-earth ion to transition metal impurities such as Cue+, Fe2+,
Co{ and Nizi. Impurities with high-energy vibrational modes (for example, OH' and H2O) can also quench the excited
state of the rare-earth ion via multi-phonon relaxation. Additionally, impurities can also directly absorb at the pump
wavelength causing heating in the form of background absorption. A great number of successful cooling cycles (each
extracting only a few kT of heat) are required to compensate for each non-radiative heating event. Thus, is it imperative
to reduce these impurities to extremely low levels.
The figure of merit for sample quality comparison we have chosen is the zero crossing temperature (ZCT), that is, the
minimum temperature at which the sample can sustain laser-induced cooling. The experiment we have developed,
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Patterson, Wendy M; Hehlen, Markus P; Epstein, Richard I & Sheik-bahae, Mansoor. Synthesis and evaluation of ultra-pure rare-earth-coped glass for laser refrigeration, article, January 1, 2009; [New Mexico]. (https://digital.library.unt.edu/ark:/67531/metadc926868/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.