Radiation-balanced lasers (RBL) combine solid-state optical refrigeration and lasing in one material to enable a net zero thermal load that allows for favorable scaling to high laser powers. A high-performance RBL material, therefore, has to first qualify as a high-performance laser-cooling material. This necessitates exquisite material purity in order to achieve the required near-unity external quantum efficiency and low background absorption. Solvent extraction, ion exchange, and electrochemical treatment of aqueous solutions or melts are some of the techniques available for the purification of starting materials used in the growth of RBL crystals. Scaling these methods to the 100s of gram scale needed for traditional Czochralski crystal growth while maintaining parts-per-billion level impurity concentrations however has proven challenging in several past efforts. In contrast, we have previously shown solvent extraction and electrochemical treatment to be effective on the several gram scale. This creates a need for exploring alternative methods for growing optical-cooling-grade fluoride crystals on the small scale. We will present results on growing Yb-doped YLiF4 (YLF) and LuLiF4 (LLF) single crystals using the vertical Bridgman method. The external quantum efficiency and background absorption of these samples will be reported and discussed in the context of RBL.
Markus P. Hehlen, Azzurra Volpi, Junwei Meng, Alexander R. Albrecht, and Mansoor Sheik-Bahae, "Bridgman growth of fluoride crystals for radiation-balanced lasers (Conference Presentation)," Proc. SPIE 10550, Optical and Electronic Cooling of Solids III, 1055007 (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2284732.5751542935001.
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