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

回音壁模式（wgm）微谐振器由于其对环境变化的响应，如磁场、电场或重力场、加速度、温度或周围介质的折射率，已被证明是各种类型的高精度传感的通用平台。接近标准量子极限的灵敏度已被证明为光机械位移和力传感器。本论文介绍了WGM谐振器的基本特性和效应，概述了近场传感的原理，以及最近实验证明的非线性Kerr诱导对称破缺的WGM谐振器，理论表明，在对称破缺区域的边缘，微谐振器的近场灵敏度增加。本论文已通过实验解决了这一研究问题，提出了初步的灵敏度结果。实验研究了对称泵浦的高Q石英棒谐振器在WGM倏逝场中尖钨针尖扰动下的谐振腔响应。钨针尖与微谐振器的距离由纳米级精密压电台精确控制。所报道的实验受到几个慢噪声源的影响，因此对尖端进行调制以研究腔响应。计算了位移谱密度，在5s时测得的最低噪声差为0.65nmHz-1/2。信噪比均在20dB以上。所有测量中的本底噪声似乎都遵循1/f线，这表明较高的调制频率可能会由于本底噪声较低而提高灵敏度和信噪比。本报告中提出的结果表明，它主要是散射和折射率变化引起的。实验，以确定从每个效果的贡献进行了概述。未来的工作还包括量化对称性破缺区域以外的灵敏度，以与这里报道的灵敏度进行比较。

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.