Laser (Cooling) Refrigeration in Erbium Based Solid State Materials
The objective of this study was to investigate the potential of erbium based solid state materials for laser refrigeration in bulk material. A great deal of work in the field has been focused on the use of ytterbium based ZBLAN glass. Some experiments have also reported cooling in thulium based solid state materials but with considerably less success. We proposed that erbium had many attractive features compared to ytterbium and therefore should be tried for cooling. The low lying energy level structure of erbium provides energy levels that could bring obtainable temperatures two orders of magnitude lower. Erbium transitions of interest for cooling fall in the near IR region (0.87 microns and 1.5 microns). Lasers for one of these transitions, in the 1.5 micron region, are well developed for communication and are in the eye-safe and water and atmosphere transparent region. Theoretical calculations are also presented so as to identify energy levels of the eleven 4f electrons in Er3+ in Cs2NaYCl6:Er3+ and the transitions between them. The strengths of the optical transitions between them have been calculated. Knowledge of such energy levels and the strength of the laser induced transitions between them is crucial for understanding the refrigeration mechanisms and different energy transfer pathways following the laser irradiation. The crystal host for erbium was a hexa-chloro-elpasolite crystal, Cs2NaYCl6:Er3+ with an 80% (stoichiometric) concentration of erbium. The best cooling results were obtained using the 0.87 micron transition. We have demonstrated bulk cooling in this crystal with a temperature difference of ~6.2 K below the surrounding temperature. The temperatures of the crystal and its immediate surrounding environment were measured using differential thermometry. Refrigeration experiments using the 1.5 micron transition were performed and the results are presented. The demonstrated temperature difference was orders of magnitude smaller. Only a temperature of ~0.015 K below the temperature of the surrounding environment was observed in this case. These results are in agreement with another group’s that has observed cooling, though a slightly poorer temperature difference, using this transition of erbium (Condon et. al., 2009). Cooling was also attempted in the 0.87 micron transition of another crystal host, KPb2Cl5:Er, which has a concentration of about one percent of erbium. We did not observe any cooling in this crystal. However, the first cooling reports in erbium based systems were with this crystal where another group observed cooling by 0.7 K using the same transition (Fernández, García-Adeva, & Balda, 2006).
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