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Cooling and sensing using whispering gallery mode resonators

Li, YL; (2016) Cooling and sensing using whispering gallery mode resonators. Doctoral thesis , UCL (University College London). Green open access

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This thesis reports on a detailed exploration of the optomechanical interaction between a tapered optical fibre and a silica microsphere mounted on a cantilever. The amount of light evanescently coupled from the fibre into the optical whispering gallery mode of the sphere is exquisitely sensitive to their separation allowing fast measurement of picometre displacements of both the microsphere-cantilever and the fibre. By exploiting this enhanced transduction, strong active feedback damping/cooling of the thermal motion of both the fibre and microsphere-cantilever have been demonstrated to the noise limit of the system. The cavity enhanced optical dipole force between the fibre and the sphere was used to damp multiple mechanical modes of the tapered fibre, while a piezo-stack at the clamped end of the microsphere-cantilever allowed for cooling of its centre-of-mass motion and the second mechanical eigenmode. The effect of noise within the feedback loop was shown to invert the measured mechanical mode spectrum at high feedback gain as the noise itself is fed into the resonator. A rich variety of feedback induced spring stiffening and softening of the mode is measured when time delays are introduced. Cooling of the mechanical modes of the taper, which are ubiquitous to many whispering gallery mode experiments and are considered as unwanted noise, has not been achieved previously. Simultaneous operation of both feedback schemes was demonstrated for the first time, providing stabilization of the system. By using the microsphere-cantilever as an inertial test mass, measurement of its displacement induced by acceleration can resolve micro-g accelerations at high bandwidth.

Type: Thesis (Doctoral)
Title: Cooling and sensing using whispering gallery mode resonators
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
Keywords: Whispering gallery mode, tapered fibre, optomechanics, cavity, laser, cooling, microsphere, cantilever
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1522162
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