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Rivière, Rémi (2011): Cavity optomechanics with silica toroidal microresonators down to low phonon occupancy. Dissertation, LMU München: Fakultät für Physik



In this thesis, I report on the cooling of a macroscopic harmonic mechanical oscillator of mass on the order of 10ng close to its quantum ground state. To perform the refrigeration, we exploit the optomechanical interaction that couples the mechanical degree of freedom to an optical cavity mode via the light's radiation pressure. The delayed response of the intracavity field upon mechanical vibration leads to a viscous intracavity radiation pressure force responsible for the dynamical backaction cooling, as is theoretically introduced in chapter 1. In chapter 2, we review the experimental system accommodating this process: the silica microtoroidal cavity. It advantageously hosts a significant optomechanical coupling between the supported high-finesse (close to 10^6) optical whispering-gallery modes and the mechanical radial breathing mode oscillating at radio frequencies (tens of MHz). In chapter 3, we detail the experimental efforts performed to improve the effect of the cooling on the system and thus to reach a lower average number of mechanical energy quanta, or phonons. The various sources of mechanical dissipations are studied. Their magnitude is diminished by optimizing the mechanical structure, therefore reducing the coupling of the mechanical mode to its warm thermal environment. In the newly developed spoke-anchored toroidal microcavities, engineering the intermode coupling minimizes the system’s damping down to the limit imposed by the properties of the vitreous silica material. To reduce the temperature of the environment itself, the experiment is pre-cooled first in a prototype helium-4 cryostat. This enables the observation of novel dispersive optical properties of fused silica and the study of the sample's thermalization at cryogenic temperatures. To further increase the pre-cooling, the setup is finally implemented in a colder helium-3 cryostat operated at 850mK. Using the balanced homodyne interferometer constructed to detect the mechanical vibration with quantum-limited sensitivity, we report on the cooling performed in the resolved-sideband configuration that is fundamentally required to reach the ground state. A mean phonon occupancy of 9 +/-1 is achieved. The fact that only simple technical problems limit further cooling proves that the developed experimental system is finally optimized for revealing quantum signatures of a macroscopic mechanical oscillator cooled by dynamical backaction. Finally, the effect of the optomechanical interaction on the optical properties of the cavity is measured and analyzed, leading to the first observation of optomechanically induced transparency. This constitutes an experimental manifestation of the mutual character of interaction between light and mechanical motion.