Püschel Stefan, Mauerhoff Felix, Kränkel Christian, Tanaka Hiroki
Opt Express. 2022 Dec 19;30(26):47235-47248. doi: 10.1364/OE.472633.
Laser cooling by anti-Stokes fluorescence is a technology to realize all-solid-state optical cryocoolers. We grew Yb-doped KYF (Yb:KYF) crystals as novel laser cooling media and compare their cooling performance to Yb-doped LiYF (Yb:YLF) crystals also grown in our institute. We present temperature-dependent absorption and emission cross sections as well as the fluorescence lifetime of Yb:KYF, and calculate its material figure-of-merit for laser cooling. Yb:KYF exhibits a higher figure-of-merit than Yb:YLF at temperatures below 200 K. This is because, in contrast to Yb:YLF, the excitation transition from the second-highest Stark level of the ground state is best-suited for cryogenic cooling in Yb:KYF. Thus, it has the potential to achieve unprecedentedly low temperatures below the boiling point of liquid nitrogen. In this work, we observe the first laser cooling of Yb:KYF, and obtain a background absorption coefficient of ∼10 cm, which is among the lowest ever reported for Yb-doped fluoride crystals. A simple model calculation predicts that our Yb:KYF and Yb:YLF crystals can potentially be cooled down to ≈100 K in a high-power cooling setup. Our Yb:KYF crystals still leave room for further improvement through the optimization of the growth process and the use of purer raw materials.
通过反斯托克斯荧光进行激光冷却，是一种实现全固态光学制冷机的技术。我们生长了掺镱的KYF（Yb:KYF）晶体作为新型激光冷却介质，并将其冷却性能与我们研究所生长的掺镱LiYF（Yb:YLF）晶体进行比较。我们给出了Yb:KYF随温度变化的吸收和发射截面以及荧光寿命，并计算了其用于激光冷却的品质因数。在200 K以下的温度下，Yb:KYF的品质因数高于Yb:YLF。这是因为，与Yb:YLF不同，从基态第二高斯塔克能级的激发跃迁最适合Yb:KYF中的低温冷却。因此，它有潜力实现低于液氮沸点的前所未有的低温。在这项工作中，我们观察到了Yb:KYF的首次激光冷却，并获得了约10 cm的背景吸收系数，这是掺镱氟化物晶体所报道的最低值之一。一个简单的模型计算预测，在高功率冷却装置中，我们的Yb:KYF和Yb:YLF晶体有可能被冷却到≈100 K。通过优化生长工艺和使用更纯的原材料，我们的Yb:KYF晶体仍有进一步改进的空间。
