News | 09-11-2022

IKZ team develops novel fluoride crystal for laser cooling of solids

IKZ researchers identify a novel crystalline material to boost all-solid-state optical cryocoolers.

Multi-pass laser excitation within a Yb:YLF single crystal; the green luminescence is originating from the small concentration of residual impurities in the sample.

Laser cooling of solids by anti-Stokes fluorescence is a unique technology enabling noncontact compact, lightweight, and vibration free cooling systems, so-called all-solid-state optical cryocoolers. Special crystals get cooled by laser radiation and can take the heat from attached materials, even cooling them down to cryogenic temperatures (T < 120 K). These features are particularly suited for cooling systems implemented in space satellites. Since 2020, the IKZ junior research group “Fluoride Crystals for Photonics” headed by Dr. Hiroki Tanaka and including PhD student Stefan Püschel has been working on this topic by an interdisciplinary approach involving both crystal growth and characterization of application-specific properties.

So far, the scientific community has been mainly using Yb-doped YLiF4 (YLF). In this research, introducing new materials is challenging as usable crystals must be virtually defect free in regard to both structural imperfections and impurities. These defects would lead to unwanted parasitic absorption and internal heating that severely limit or even prevent laser cooling.

The IKZ team has grown Yb:YLF crystals that are among the best quality ever reported for laser cooling of solids. But even further, we were able to prepare very pure Yb-doped KY3F10 (KYF) crystals that can potentially outperform Yb:YLF. The material-specific figure-of-merit for laser cooling is approximately one order of magnitude higher below 100 K. This will enable to reach unprecedentedly low temperatures beyond the key milestone of 77 K, the boiling point of liquid nitrogen. For comparison, the achievable temperature using Yb:YLF has been so far limited to about 90 K. The further reduction of achievable temperature by this technology is a decisive step towards real-world applications.

Using our new material Yb:KYF grown by the Czochralski method, we have demonstrated efficient high-power laser cooling to exceed the current state-of-the-art. The results including also measurements on ultra-pure Yb:YLF for comparison will be presented at the upcoming Advanced Solid State Lasers conference in December at Barcelona [1].


Further information:
Hiroki Tanaka (Oxide & Fluoride Section)

[1] Stefan Püschel, “Laser Cooling of an Yb3+-doped Cubic KY3F10 Single Crystal”, ATh1A.3 – presented on Thursday, 15 December, 08:30 - 08:45 am CET.