Title:
Toward Yb:YLF-based optical cryocooler: monolithic architecture, pump management, fluorescence thermometry and saturation effects

Date, Time & Location:
Friday, May 30th, 2025, at 10 am, PAIS, Room 3205

Dissertation Committee:
Dr. Alexander Albrecht (Committee Chair), Physics and Astronomy
Dr. Payman Zarkesh-Ha, Electrical and Computer Engineering
Dr. Markus Hehlen, Los Alamos National Laboratory
Dr. Tara Drake, Physics and Astronomy

Solid-state optical refrigeration using anti-Stokes fluorescence offers a compact, vibration-free route to cryogenic cooling. This dissertation advances ytterbium-doped yttrium lithium fluoride (Yb:YLF) optical refrigerators, achieving a record-low temperature of 124 K by addressing key limitations from absorption saturation. Incorporating saturation effects into the cooling efficiency model enables accurate predictions of the crystal’s cooling performance. An improved model and simulation for astigmatic Herriott cells was developed to account for high pump intensities and clipping of the pump beam, which both effects lead to a reduction in the devices cooling capability. A novel fluorescence-based thermometry method was also established, improving upon differential luminescence thermometry to calibrate the emission of Yb:YLF for temperature, orientation and the reabsorption depth. This allows not only the temperature of the crystal to be determined, but the crystal can also be used to determine the location and orientation of an attached device. Finally, a monolithic architecture was developed by directly attached mirror coatings to the crystal and using a highly divergent pump beam to uniformly fill the crystal to minimize saturation effects and create a compact device.

Jackson Kock received his Bachelor of Science degree in Physics from the University of Wisconsin–River Falls in 2014. As an undergraduate, he conducted research on optically trapped aerosols to study how a changing trapping power influenced the droplet’s position and size. He pursued his graduate studies working with Professor Mansoor Sheik-Bahae and Dr. Alexander Albrecht at the University of New Mexico in the Optical Science and Engineering program. His doctoral research advanced the ongoing development of optical refrigeration using ytterbium-doped yttrium lithium fluoride (Yb:YLF) crystals. Building upon a foundation established by prior researchers at UNM, his work contributed both experimental and theoretical advances leading to record cooling of a payload to 124 K. His key contributions include investigating the effects of absorption saturation, optimizing and developing a robust simulation of the astigmatic Herriott cell used to pump the crystal, and developing a novel fluorescence-based thermometry technique. He will have three first-author publications in peer-reviewed journals and is a regular participant in SPIE Photonics West, where he has delivered invited presentations. His pursuit of higher education and research has been made possible by the unwavering support of his loving wife and family.