Laser cooling of highly pure rare-earth-doped fluoride crystals

• Applicant: Hiroki Tanaka, Ph.D.
• Subject Area: Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 520253663

Optically active solid materials are cooled by laser excitation at a wavelength longer than the average fluorescence wavelength. As the average energy of the emitted photons is higher than the excitation photon energy – enabled by anti-Stokes fluorescence – this simple approach yields cooling of the medium and offers unique advantages: it is vibration-free, requires no moving parts or cryogenic fluids, and is potentially very compact. ‘Solid-state laser cooling’ is thus an attractive alternative to thermoelectric- and liquid-nitrogen (LN)-based cooling, and even provides the basis for ‘athermal’ lasers, which generate no heat even at the highest pump powers. This project aims to understand the role of impurities on laser cooling processes to achieve the highest cooling efficiencies and to cool beyond the boiling temperature of nitrogen. The project also aims to establish strategies to suppress detrimental impurities during crystal growth processes. These investigations will be performed in the most successful cooling materials ytterbium-doped LiYF4 (Yb:YLF) and LiLuF4 (Yb:LLF). This project involves research in crystal growth, compositional and spectroscopic investigations, as well as laser cooling characterization. The Leibniz-Institut für Kristallzüchtung (IKZ) offers an ideal environment to combine these topics as it is leading in crystal growth technology. In addition, its newly established Center for Laser Materials provides state-of-the-art equipment and know-how for optical spectroscopy and laser experiments. The applicant has a strong background in solid-state lasers and is highly experienced in fluoride crystal growth and is thus ideally suited to carry out this interdisciplinary research. In this project, we will reveal the influence of foreign rare-earth and transition-metal impurities on laser cooling processes in Yb:YLF and Yb:LLF at a wide range of temperatures. To this end, Yb:YLF and Yb:LLF crystals intentionally doped with target impurities will be grown by the Czochralski and the vertical Bridgman methods. Spectroscopic analyses reveal the interaction of Yb3+ ions with foreign rare-earth as well as transition-metal impurities. The segregation behavior of the transition-metal impurities in the vertical Bridgman growth will be also studied. The impact of impurities on the laser cooling process in Yb:YLF and Yb:LLF will be directly evaluated in a laser cooling setup. We will evaluate crystals by applying laser-induced thermal modulation spectroscopy which determines sample-specific parameters for laser cooling. We will also investigate the achievable temperature of prepared laser cooling samples under high-power laser excitation. This outcome of this project would strongly contribute to the scientific community currently facing the issue of impurities. Thus, this fundamental research project would be crucial for the further progress and application of solid-state laser cooling.

• DFG Programme: Research Grants