Laser Refrigeration of Ytterbium-Doped Alkali-Rare-Earth-Fluoride Nanostructures and Applications for Radiation-Balanced Lasers

Thesis (Ph.D.)--University of Washington, 2021This work focuses on the development of new laser cooling materials and their applications for radiation-balanced lasers. The report begins with a brief introduction in Chapter 1 to the field of solid-state laser refrigeration, emphasizing the fundamental physical phenomena that have made solid-state laser-cooling possible. The concept of radiation balance lasers and some attempts to realize radiation balanced lasing with different gain media structures are also introduced. Chapter 2 introduces non-contact thermometries for micrometer and nanometer-scale materials, especially those frequently mentioned in this dissertation. Different temperature-dependent parameters are measured to interpret temperature, such as fluorescence, diffusion coefficient, Young's modulus, etc. The widely used Er(III) ratiometric thermometry is discussed in Chapter 3 in detail. A new green emission is introduced, which overlaps with the green luminescence typically used for intensity ratiometric thermometry. This emission is often neglected and is very sensitive to excitation power. Therefore, the wavelength intervals for intensity ratio measurement must be selected carefully to achieve accurate temperature readouts. In Chapter 4, a group of new laser cooling materials, potassium lutetium fluorides, are discussed. The rapid, low-cost hydrothermal synthesis of potassium lutetium fluoride materials was performed. Four crystalline phases were synthesized, namely trigonal KLuF4, orthorhombic KLu2F7, cubic KLu3F10, and orthorhombic K2LuF5, with each phase exhibiting unique microcrystalline morphologies. Fluorescence spectra and emission lifetimes of the four crystalline phases were characterized based on the point-group symmetries of trivalent cations. Laser refrigeration was measured by observing both the optomechanical eigenfrequencies of microcrystals on cantilevers in vacuum and the Brownian dynamics of optically-trapped microcrystals in water. Among all four crystalline phases, the most significant laser cooling was observed for 10%Yb:KLuF4 with cooling of 8.57 ± 2.07 K below room temperature. The end of this chapter presents some preliminary crystal growth results of potassium lutetium fluoride with different stoichiometries. The exact growth mechanism remains unclear for future exploration. Chapter 5 discussed point defects in laser refrigeration materials and their impacts on cooling performance. After X-ray irradiation, point defects are formed in Yb:YLiF4 (YLF) microcrystals. Two defects with different thermal stability are formed according to TL spectra. These defects are assigned to F-centers tentatively based on EPR spectra. The laser cooling performance of such Yb:YLF deteriorated a lot after irradiation. This is probably due to the increased non-radiative relaxation rate with the interaction of point defects and Yb(III). In Chapter 6, applications of laser cooling crystals to radiation-balanced lasers are discussed. Although the output power of commercial fiber lasers has been reported to exceed 500 kW, the heat generated within fiber gain-media has limited the generation of higher laser powers due to thermal lensing and melting of the gain-media at high temperatures. Radiation-balanced fiber lasers promise to mitigate detrimental thermal effects within fiber gain-media based on using upconverted, anti-Stokes photoluminescence to extract heat from the optical fiber's core. In this chapter, we experimentally demonstrate that Yb(III) ions within YLF microcrystals can cool the cladding of optical fibers. Based on this experiment, a design for radiation-balanced fiber-lasers using a composite fiber cladding material is presented that incorporates YLF nanocrystals as the active photonic heat engine. YLF crystals have the potential to form composite cladding materials to mitigate thermal gradients within the core and cladding based on anti-Stokes photoluminescence. With the development of new amorphous optical refrigeration glass materials, a new design of an all-glass multi-mode clad pumped fiber laser is proposed to provide more cooling power with clad pumping. Finally, analytical models of heat transfer within the fiber of both designs are presented where the light within the fiber core is responsible for the heating while the light in the cladding excites Yb(III) ions for anti-Stokes laser refrigeration