Symmetry breaking enhanced near-field sensing with whispering gallery mode microresonators

Whispering gallery mode (wgm) microresonators have shown to be versatile platforms for various types of high-precision sensing due to their response to changes in their environment, such as magnetic, electric or gravitational fields, acceleration, temperature, or refractive index of surrounding media. Sensitivities close to the standard quantum limit have been demonstrated for optomechanical displacement and force sensors. This thesis covers the fundamental properties and effects observed in wgm resonators, outlines the principles of near-field sensing and the recently experimentally demonstrated nonlinear Kerr induced symmetry breaking observed wgm resonators with counter propagating fields.
Theory indicates increased near-field sensitivity for a microresonator on the verge of the symmetry broken regime. This thesis has experimentally addressed this research question, presenting preliminary sensitivity results. The experiment investigated the cavity response of a symmetrically pumped high-Q silica rod resonator due to a sharp tungsten tip's perturbation within the wgm evanescent field. The tungsten tip distance from the microresonator was precisely controlled by a nanometre precision piezo stage. The reported experiment was affected by several slow noise sources, and therefore the tip was modulated to study the cavity response. Displacement spectral densities were calculated, and the lowest noise poor obtained was 0.65nmHz -1/2 for a 5s duration measurement. Signal-to-noise ratios were above 20dB. The noise floor in all measurements seemed to follow a 1 /f line, indicating that higher modulation frequencies will possibly have increased sensitivity and signal-to-noise ratio due to the low noise floor.
Further experiments are needed to fully identify the dominant effect caused by the tungsten tip in the evanescent field of the resonator. The results presented in this report suggest it is primarily scattering and refractive index change induced. Experiments to determine the contributions from each effect are outlined. Future work also includes quantifying the sensitivity outside the symmetry breaking region to compare with the sensitivities reported here.

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