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Anetsberger, Georg (2010): Novel Cavity Optomechanical Systems at the Micro- and Nanoscale and Quantum Measurements of Nanomechanical Oscillators. Dissertation, LMU München: Fakultät für Physik



This thesis reports on coupling optical microresonators to micro- and nanomechanical oscillators. The mutual optomechanical coupling based on radiation pressure between the microcavity and a mechanical degree of freedom modulating its spatial structure thereby allows both transduction and actuation of the motion of the mechanical degree of freedom by the light field launched into the microcavity. The first part of the thesis reports on a novel experimental approach based on cavity enhanced evanescent near-fields of toroid microresonators. It enables the extension of dispersive cavity optomechanical coupling to sub-wavelength scale nanomechanical oscillators which are at the heart of a variety of precision measurements. The optomechanical coupling present in the developed system is carefully analyzed experimentally and good agreement with theoretical expectations is found. The demonstrated platform allows transduction of nanomechanical motion with an exceptionally high sensitivity, outperforming the previous state-of-the-art transducers. Thereby, for the first time a measurement imprecision lower than the level of the standard quantum limit is achieved. In the present measurements, quantum backaction should already be the dominating contribution to the measurement sensitivity which is however masked by thermal noise. This may pave the way to the first experimental demonstration of radiation pressure quantum backaction on a solid-state mechanical oscillator. Moreover, the radiation pressure interaction between evanescent cavity field and nanomechanical oscillator is shown to enable actuating and controlling the motional state of the oscillator. Both amplification, leading to self-sustained mechanical oscillations, and cooling by radiation pressure dynamical backaction is reported. In addition, the capability of the near-field platform to implement resonant interaction of a mechanical mode with two optical modes is shown as well as the feasibility of quadratic coupling to the nanomechanical oscillators. In the second part of the thesis monolithic on-chip resonators that combine ultra-low optical and mechanical dissipation are designed. To this end, the intrinsic mechanical modes of toroid microresonators are analyzed in detail. High-sensitivity measurements enable the observation of a plethora of mechanical modes and good agreement with finite element modelling is found. In particular the dissipation mechanisms limiting their mechanical quality are studied. Clamping losses are identified as the dominant loss mechanism at room temperature. Using a novel geometric design, these are systematically minimized which leads to spoke-supported microresonators with intrinsic material-loss limited mechanical quality factors rivalling the best published values at similar frequencies.