Cavity optomechanics with silica toroidal microresonators down to low phonon occupancy

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.