Hydrothermal Synthesis and Solid-State Laser Refrigeration of Ytterbium-Doped Potassium Lutetium Fluoride (KLF) Microcrystals

Abstract

Fluoride crystals, due to their low phonon energies, are attractive hosts of trivalent lanthanide ions for applications in upconverting phosphors, quantum information science, and solid-state laser refrigeration. In this article, we report the rapid, low-cost hydrothermal synthesis of potassium lutetium fluoride (KLF) microcrystals for applications in solid-state laser refrigeration. Four crystalline phases were synthesized, namely orthorhombic K<sub>2</sub>LuF<sub>5</sub> (Pnma), trigonal KLuF<sub>4</sub> (P3<sub>1</sub>21), orthorhombic KLu<sub>2</sub>F<sub>7</sub> (Pna2<sub>1</sub>), and cubic KLu<sub>3</sub>F<sub>10</sub> (Fm3m), with each phase exhibiting unique microcrystalline morphologies. Luminescence spectra and emission lifetimes of the four crystalline phases were characterized based on the point-group symmetry of trivalent cations. Laser refrigeration was measured by observing both the optomechanical eigenfrequencies of microcrystals on cantilevers in vacuum, and also the Brownian dynamics of optically trapped microcrystals in water. Among all four crystalline phases, the most significant cooling was observed for 10%Yb:KLuF<sub>4</sub> with cooling of 8.6 $\pm$ 2.1 K below room temperature. Reduced heating was observed with 10%Yb:K<sub>2</sub>LuF<sub>5</sub>

Microcrystal alignment in drawn fiber over sub-meter to kilometer length scales

“…While both the rod diameter and inner diameter of the surrounding tube were nominally 3/8 th inch, caliper measurements indicated the actual rod diameter closer to 9.30 mm and inner diameter of the tube ≈ 9.65 mm. Ytterbium-doped potassium lutetium fluoride (30% Yb:K 2 LuF 5 , KLF) microcrystals [20] were suspended in grapeseed oil, and this mixture was filled into the thin gap between the polymer rod and surrounding tube of the preform. For the idealized case of a preform rod positioned concentrically within the surrounding tube, the liquid suspension of microcrystals in the gap is approximately 180 microns wide as shown in Fig.…”

Optical Trapping of High-Aspect-Ratio NaYF Hexagonal Prisms for kHz-MHz Gravitational Wave Detectors

“…Since the first experimental demonstration of cold Brownian motion [29], solid-state laser refrigeration [30] has proven to be an effective way of preventing detrimental photothermal heating of optically-levitated materials. In addition, NaYF and other rare-earth-doped crystals have been studied before in the context of optical vacuum levitation [31,32] and have been shown to exhibit laser refrigeration when trapped with light of an appropriate wavelength.…”

“…By choosing such a material for the sensor, in the future laser refrigeration methods may enable higher trapping intensity and higher trap frequencies for gravitational wave searches approaching the several hundred kHz frequency range. To produce multi-µm-scale high-aspect ratio particles relevant for GW detectors, we recently developed a novel hydrothermal synthesis [34] to drive growth of the hexagonal beta NaYF into thin, wide plates that was not previously possible [30].…”