Coherent suppression of backscattering in optical microresonators

As light propagates along a waveguide, a fraction of the field can be reflected by Rayleigh scatterers. In high quality-factor whispering-gallery-mode microresonators, this intrinsic backscattering is primarily caused by either surface or bulk material imperfections. For several types of microresonator-based experiments and applications, minimal backscattering in the cavity is of critical importance, and thus the ability to suppress the backscattering is essential. We demonstrate that introducing an additional scatterer in the near-field of a high-quality-factor microresonator can coherently suppress the amount of backscattering in a microresonator by more than 30 dB. The method relies on controlling the scatterer's position in order for the intrinsic and scatterer-induced backpropagating fields to destructively interfere. This technique is useful in microresonator applications where backscattering is currently limiting the performance of devices, such as ring-laser gyroscopes and dual frequency combs that both suffer from injection locking. Moreover, these findings are of interest for integrated photonic circuits in which backreflections could negatively impact the stability of laser sources or other components

Near-field-scattering-based optical control and Brillouin optomechanics in optical microresonators

Resonance is a powerful effect that occurs throughout nature. For example, the effect is key to the excitement of playground swings and it underpins technologies ranging from musical instruments to atomic clocks. In optics, microresonators are extensively used to provide such enhancement and are employed in a number of areas including sensing, metrology, optomechanics, and quantum optics to name a few prominent examples. This thesis comprises two main parts. One part expands the optical microresonator control toolbox by demonstrating suppression of backscattering. The other part uses a whispering-gallery-mode microresonator for resonant enhancement of a Brillouin optomechanical interaction to prepare and characterise non-Gaussian mechanical states. The first part explores a technique for coherently controlling backscattering in microresonators by introducing a sub-wavelength-size scatterer within the near field of the resonator. The scatterer's position determines the phase and amplitude of the induced backscattering, and by tuning its position, destructive interference between the induced and intrinsic backscattering can reduce unwanted optical back reflections. The presented experiment demonstrates a suppression exceeding 34dB of the intrinsic backscattering level, limited by photodetector noise. The technique can be applied to experiments where backscattering is currently limiting performance, such as optical gyroscopes. The second part of this thesis presents an experiment preparing non-Gaussian states of mechanical motion via heralded single- and double-phonon subtraction from a laser-cooled thermal mechanical state. The experiment utilises a combination of single-photon detection for heralded state-preparation, and heterodyne detection for verification and characterisation of the prepared states. The work advances the state of the art for optics-based tomography of mechanical states by showing more than one order of magnitude improvement in the s-parameter, which captures the effects of measurement inefficiencies and added noise in tomography and state reconstruction experiments. Further improving the measurement efficiency provides a path towards tomography of non-classical mechanical states via optomechanics.Open Acces

Thermal Conductivity, Heat Sources and Temperature Profiles of Li-ion Batteries

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