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Title:Laser cooling of solids through inelastic light scattering
Author(s):Chen, Yin-Chung
Director of Research:Bahl, Gaurav
Doctoral Committee Chair(s):Bahl, Gaurav
Doctoral Committee Member(s):Eden, Gary; Choquette, Kent; Toussaint, Kimani; Schleife, André
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Laser cooling
Raman scattering
Brillouin scattering
Photonic Crystals
Abstract:The laser cooling of vibrational states of solids has been achieved through photoluminescence in rare-earth elements and semiconductors, optical forces in optomechanics, and Brillouin scattering opto-acoustic interaction. Among all these approaches, photoluminescence or fluorescence cooling has had the greatest success in cooling materials down to cryogenic temperatures. However, cooling of solids through fluorescence relies on specific electronic states in the system, and to date has only been achieved in rare-earth elements, such as Yb3+, Tm3+, Er3+, and Ho3+. More recently, fluorescence cooling has also been demonstrated experimentally in CdS nanobelts. Nevertheless, such a process remains heavily material dependent. In this dissertation I provide an analysis on a more general approach to laser cooling of solids through inelastic light scattering, which includes Brillouin and Raman scattering. In both Brillouin and Raman scattering, the incident light either absorbs (anti-Stokes) thermal energy from or releases (Stokes) thermal energy to the system in the form of phonons. Therefore, they can both be used for laser cooling of solids. I first show analytically that photonic density of states (DoS) engineering can address the two fundamental requirements for achieving spontaneous Raman cooling: suppressing the dominance of Stokes (heating) transitions and the enhancement of anti-Stokes (cooling) efficiency beyond the natural optical absorption of the material. I develop a general model for the DoS modification to spontaneous Raman scattering probabilities, and elucidate the necessary and minimum condition required for achieving net Raman cooling. With a suitably engineered DoS, I establish the enticing possibility of the refrigeration of intrinsic silicon by annihilating phonons from all its Raman active phonon modes simultaneously, through a single telecom wavelength pump. This Raman cooling analysis is then generalized to incorporate the full isotropic features of Raman scattering. I consider the influence of both anisotropic photonic density of states and the intrinsically anisotropic scattering pattern from the Raman selection rules. I demonstrate an optimization of the Raman cooling figure of merit considering all possible orientations for the material crystal for two example photonic crystals. The results show that the anisotropic description of the photonic density of states and the optimization process is necessary to obtain the best Raman cooling efficiency. In addition to Raman cooling, the Brillouin cooling process is also generalized to a linear waveguide system. The analysis establishes the conditions under which Brillouin cooling of phonons of both low and high group velocities may be achieved in a linear waveguide, and reveals the key regimes of operation for the process. The calculations indicate that measurable cooling may occur in a system having phonons with spatial loss rate that is of the same order as the spatial optical loss rate. The thermodynamics of laser cooling processes through inelastic light scattering is then investigated. The analysis indicates that thermodynamics does not restrict the enhancement of cooling efficiency through photonic DoS engineering, and further confirms the broad applicability of the theory. Lastly, I present the design principles and experimental measurements of aluminum nitride on-chip photonic devices for Raman laser cooling. The results point to further improvement of the design of photonic circuit for future laser cooling experiments. Since laser cooling of solids through inelastic scattering is relatively independent of material properties, it can be easily applied to improve other laser cooling methods by imposing additional photonic structures to the systems. It also provides solutions to some of the unresolved laser cooling challenges, such as laser cooling of indirect band gap semiconductors. Such a broadly applicable method for laser cooling can greatly impact our ability to control the thermal states of matter.
Issue Date:2018-11-27
Rights Information:Copyright 2018 Yinchung Chen
Date Available in IDEALS:2019-02-07
Date Deposited:2018-12

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