Synthesis and Characterization of Novel Sodium Yttrium Fluoride (NaYF) Morphologies for Laser Refrigeration and Levitated Optomechanics Applications

Abstract

Lanthanide-doped optical materials are a cornerstone of modern technologies, from artificial lighting, high-definition displays, security printing and telecommunication to bioimaging, targeted drug delivery, and optical sensors. Furthermore, they are ubiquitous in the natural sciences, where they continue to push the frontiers of research. Especially, high precision sensing applications will benefit from upconversion, photon avalanching, laser refrigeration or the quantum capabilities of lanthanide ions doped into host materials. While these alone open avenues towards unprecedented sensitivities, the combination of lanthanide optics and levitated optomechanics holds further promise for breakthrough discoveries and technological advances. However, novel applications and experiments require unique material properties and distinct morphologies, necessitating the continuous advancement of synthesis techniques.This work focuses on lanthanide doped sodium yttrium fluoride (NaYF), the most prevalent upconversion host material and one of the leading candidates for photon avalanching, lanthanide quantum information sciences and anti-Stokes laser refrigeration. Chapter 1 covers the basics of lanthanide optics, its applications, the role of the host material and the properties of NaYF as well as optical levitation. The optical levitation of sensors holds great promise for the development of gyroscopes, force and pressure sensing methods and was proposed for the detection of high-frequency gravitational waves. In order to achieve the latter goal, massive, two-dimensional particle morphologies are required. Chapter 2 presents the hydrothermal synthesis of β-NaYF disks with large diameters and ultra-high aspect ratios based on methylimidodiacetic acid (MIDA) as organic additive. The synthesis developed here increases the available size range of β-NaYF disks by a factor of more than seven reaching diameters of up to 44 µm. This enabled the observation of twinning using single crystal X-ray diffraction experiments. Furthermore, the laser refrigeration of 4.9 K by one of six individual microcrystals based on ytterbium anti-Stokes photoluminescence is presented. While the magnitude and percentage of cooling crystals is low, the method presented here holds promise for the development of laser refrigerated microoptics and levitated sensors. A common cause for the low optical quality of NaYF is the abundance of OH– defects, which are more prevalent in hydrothermal materials. Thus, there is a need to mitigate their formation to improve quantum yields and laser cooling magnitude as well as efficiency. Chapter 3 presents an approach based on the substitution of H2O with D2O in the synthesis of β-NaYF. The deuterothermal microdisks outperformed both literature and samples grown in H2O by cooling up to 15 K. Although the cooling efficiency is improved, unexpected interactions with the gas phase resulted in a decline over time that warrant further investigation. The mitigation of OH– defects in lanthanide-doped optical materials has the potential to drastically improve a multitude of optical phenomena beyond laser refrigeration and sodium yttrium fluoride host materials. The development of quasi-2D β-NaYF disks culminates in the combination of lanthanide optics and levitated optomechanics presented in Chapter 4. Rotational control of individual optically levitated Er:β-NaYF disks was achieved by altering the pump wavelength, which allowed the embedding of a bitwise binary sequence into the particle dynamics. The incorporation of erbium will further enable in situ thermometry, temperature control, and background free detection of particle motion in future experiments. The sensor developed here holds promise for pressure sensing and gyroscopes. Furthermore, long-term rotational dynamics qualitatively consistent with tennis-racket-like (Dzhanibekov) motion were observed and could lead to significant quantum applications. Chapter 5 focuses on the development of another levitated sensor system, namely a composite of NV– nanodiamonds and α-NaYF. NV– centers are a workhorse in quantum information sciences, including applications in computing and sensing, but optically levitated nanodiamonds are susceptible to photothermal heating. The combination with a laser refrigeration material holds promise for overcoming this problem. Here, a synthetic method to obtain nanodiamonds permanently attached to or incorporated into an α-NaYF shell, within the size range for single beam optical levitation is presented. Although thermometry based on both NV– and Yb3+ PL indicated thermal coupling, no laser refrigeration was observed for clusters of the composite. The developed method nevertheless builds a foundation for the prospect temperature control of quantum relevant sensors embedded within laser refrigeration materials or could be used to introduce other optical lanthanide functionalities. In Chapter 6 two novel NaYF syntheses using sodium dodecyl sulfate (SDS) are explored. The first allows for the growth of β-NaYF nanocrystals at room temperature using sonochemistry. The optical quality of the β-NaYF obtained is not competitive to more established synthetic methods, but the capability of growing β-NaYF without organic solvents or external heating using a high-throughput method holds promise for on demand optical applications that do not require near unity quantum yields. The latter part of Chapter 6 presents the single step hydrothermal synthesis of porous α-NaYF using SDS. The enhanced environmental interactions of this morphology are leveraged to study the influence of the gas phase on anti-Stokes laser refrigeration. A pressure-dependent switch between cooling and photothermal heating is demonstrated and indicates that gas phase properties, like the molecular mass as well as the number of rotational and vibrational degrees of freedom and accommodation coefficient, need to be considered for laser refrigeration on the nanoscale. The α-NaYF nanocubes are furthermore optically levitated, and their porosity was studied on the single particle level. This holds significant promise for the development of a novel class of porous sensors with applications such as extreme vacuum sensing. Lastly, Chapter 7 discusses significant contributions from the work presented here and points toward prospect research directions.