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.