Solid-State Laser Refrigeration at GPa Pressures

Although solid-state laser-refrigeration recently has been demonstrated to reach cryogenic temperatures in vacuum, to date the solid-state laser refrigeration of materials at elevated pressure conditions has remained unexplored. Here we demonstrate the laser cooling of ytterbium-doped yttirum-lithium-fluoride (10%Yb³⁺:YLiF₄, or Yb:YLF)

>17K below room temperature at pressures >4 GPa in a diamond anvil cell using lithium fluoride and ice-VII as a quasi-hydrostatic pressure media. Temperature measurements are quantified using a ratiometric-thermometry approach involving a Boltzmann fit to excited states distribution through 4f-4f Stark-level transitions from the Yb³⁺ ions that occur between the ²F_5/2 and ²F_7/2 manifolds. At pressures between 7 and 12 GPa the YLF grains are observed to undergo a martensitic phase transition from a tetragonal scheelite phase (space group I41/a, Z = 4, No. 88) to a monoclinic fergusonite phase (space group I2/a, Z = 4, No. 15) which modifies the crys-

tal field splitting of the ground- and excited- state manifolds, but is observed to not eliminate laser cooling. Solid-state laser refrigeration at extreme pressures could allow researchers to use rapid photothermal cycling to explore temperature-dependent properties of materials, including electronic-structure phase-transitions, without the need for external cryostats.