In Situ Solid State Laser Refrigeration at GPa Pressures

Solid state laser refrigeration can cool a host lattice through the emission of anti-Stokes upconverted photons from rare-earth dopant ions. Recently, laser refrigeration has also been demonstrated in condensed phases, including liquid water¹ using single-beam laser tweezers. Laser refrigeration within a diamond anvil cell would allow researchers to explore in situ temperature-dependent properties of materials without external hardware. To date, laser refrigeration at extreme pressures conditions has not yet been demonstrated. Here, we demonstrate the cooling of 10%Yb³⁺:LiYF₄ (Yb:YLF, I₄₁/a space group) micro-crystals at elevated pressures in a diamond anvil cell. In addition, we investigate the impact of a scheelite to fergusonite phase transition on laser refrigeration at pressures >10.5 GPa. Varying the irradiance of a 1020nm continuous wave laser shows that Yb:YLF can be effectively cooled with a linear dependence on laser irradiances. Cooling temperatures are quantified using a ratiometric thermometry approach involving a Boltzmann fit to f-f transitions from the Yb³⁺ ions that occur between the Stark levels within the ²F_5/2 and ²F_7/2 manifolds.
1. Roder, P.B. et al. (2015) Laser refrigeration of hydrothermal nanocrystals in physiological media. PNAS 112: 15024-15029