Synthesis, Applications, and Optical Properties of Rare Earth Fluorides for Laser Refrigeration

Abstract

One of the most powerful scientific tools available to studying natural phenomena is light. For spectroscopic analysis, Light has uses in nearly every branch of science, yet light has the often unintended side effect of causing photothermal heating. This heating will alter the system being measured, which can cause serious issues for sensitivity and accuracy of measurement readouts. It becomes necessary to offset this heating in many applications. While this can be done with traditional methods, mainly heat sinks, cryogenic fluids, and thermoelectric coolers, these also add constraints to the system by requiring cooling via conduction with a cooler medium. One solution is the use of laser refrigeration to remove the excess energy from the system via radiation through the emission of light. Chapter 1 focuses on understanding laser refrigeration, as well as understanding optical tweezers, which stand to gain much from the use of laser refrigeration. This work focuses on understanding the current scientific body of knowledge surrounding rare earth fluoride microcrystals for laser refrigeration applications, including synthesis, characterization, and applications in complex experiments. These materials represent a non-contact method of cooling micro and nano scale materials through laser irradiation alone, including the cooling of payloads attached to the surface of the crystals. In chapter 2, we discuss the synthesis of these materials. By controlling the synthesis, we can make microscale materials that cannot be grown in bulk with current methodologies. These novel crystals are predicted to have the highest possible cooling power of any material that can be reliably made. Not only can these materials be grown through these novel methods, but the morphology can be tuned easily to provide optical cavities with resonant properties, with respect to the cooling laser beam. By doing so, it becomes possible to make microcavity materials for optimized laser refrigeration through direct hydrothermal synthesis. Understanding the synthesis of these materials is critical to successfully modeling and improving upon design. The nucleation of sodium yttrium fluoride, the main material of interest, was not well understood prior to these works. We study the nucleation and find it to be a multi-step crystallization mechanism that operates through spinodal decomposition of the principal reagents into ion-rich and ion-poor phases, causing the product to crash out as a gel. The gel then reacts with the water surrounding it to transform from an amorphous YF3 phase to a crystalline NaYF phase by taking in sodium ions from the surrounding environment. This allows the gel to condense into nanocrystals of the stable, cubic alpha phase of NaYF. When heated in aqueous conditions, the alpha phase can convert to the hexagonal beta phase of NaYF. This phase of NaYF has been predicted to be exceptionally good at laser refrigeration, but it cannot be grown in bulk at high temperatures due to fracturing of bulk crystals during the melt-growth process. The high temperatures needed for crystal melts are avoided by use of hydrothermal synthesis. While this cannot grow bulk sized crystals, we can reliably grow beta phase NaYF as nanowires. This was the only morphology shown to cool prior to my work. The crystals preferentially grow along the [0001] plane, leading the long, thin nanowires. By controlling the synthesis through ligands and relative ionic concentrations, the morphology can be altered to produce hexagonal platelets up to 10 µm across while remaining less than 200 nm thick. The aspect ratio is tunable with relative reagent concentration, allowing for precise morphology control. In addition to these crystals being morphologically controllable, they are capable of undergoing laser refrigeration. By doping with 10\% Yb 3+ during the synthesis, the crystals become capable of emitting higher energy light than they absorb with sufficient efficiency to create a net loss of energy in this system, resulting in cooling, which we explore in chapter 3. The crystals are placed onto cadmium sulfide nanoribbons for thermal isolation from a large substrate, and then they are irradiated with a 1020 nm laser to induce cooling. The luminescence is collected and used for ratiometric thermometry, where specific crystal field levels are integrated for comparing emission from the different crystal field levels. The ratio of the relative emission can be calibrated against temperature, confirming that the hexagonal platelet beta phase of NaYF is capable of cooling up to 12.5 K. Further cooling may be possible with synthetic and experimental refinement. Further experiments are done with nanodiamonds attached to the surface of rare earth fluorides to act as a payload. Since nanodiamonds do not cool under NIR light, we can use them as a thermometer to determine if the cooling power of the crystal is sufficient to overcome any photothermal heating of the nanodiamonds. Debye Waller factor thermometry and ODMR thermometry confirm the nanodiamonds to be cooling, with the diamonds on YLF crystals being cooled by more than 30 K and the diamonds on NaYF crystals confirmed to cool 12.5 K below room temperature. We also explore the possiblity of cooling diamond in chapter 3 through the conversion of NV- centers into H3 centers, which can undergo anti-Stokes luminescence necessary for laser refrigeration. While the diamonds are not cooled below room temperature, we report progress in offsetting photothermal heating in NV-rich diamond samples. While the process of laser refrigeration is novel on its own, it is enhanced by applying it to other optical systems. The system most poised to take advantage of laser refrigeration is optical tweezers. In chapter 4, we discuss the use of laser refrigeration to cool objects trapped in optical tweezers. Due to the spatial isolation provided by optical tweezers, it is easy for photothermal heat to build in the absence of a method to conduct the heat away. By using materials with laser refrigeration, we can offset this heat and measure particles without the threat of vaporization of the sample. We use alpha phase NaYF crystals in a set of optical tweezers to determine the minimum possible temperature for these crystals in vacuum tweezers, reaching temperatures as low as 241 K. We also investigate the use of dual-beam tweezers for trapping hexagonal beta phase NaYF. While no cooling was investigated during these experiments, we do demonstrate the stable trapping of the hexagonal platelets with wide aspect ratios. The provide a significant improvement in materials for levitated sensor disc applications due to the high laser irradiances they can withstand. In chapter 5, we explore the other projects I have been able to participate in, including the laser refrigeration of YLF at extreme pressures through the use of a diamond anvil cell. The high pressures (more than 6 GPa) causes the YLF to undergo a phase change from a scheelite phase to a fergusonite phase, which alters the symmetry of the crystal lattice. This in turn affects crystal field splittings, which will affect the laser refrigeration. Even after the phase change has affected the crystal phase, laser refrigeration is still observed, though the magnitude is greatly diminished, going from more than 20 K below room temperature to only a few degrees. In this chapter, we also explore the generation of optical cavities through the deposition of chalcogenides via pulsed laser deposition. Quantum dots are coated on the deposited films to create cavities with quantum dots between the layers. We characterize and report the survival of the quantum dots through the pulsed laser deposition process. Lastly, chapter 6 discusses significant conclusions from the experiments presented and discuss future work in the field of laser refrigeration.