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

回音廊模式(WGM)微谐振器因其对周围介质的磁场、电场或重力场、加速度、温度或折射率等环境变化的响应，已被证明是各种类型高精度传感的通用平台。光学机械位移和力传感器的灵敏度已被证明接近标准量子极限。本文介绍了WGM谐振器的基本性质和观察到的效应，概述了近场传感的原理，以及最近实验证明的具有反向传播场的WGM谐振器的非线性克尔诱导对称性破缺。理论表明，处于对称性破裂区边缘的微谐振器的近场灵敏度增加。本论文对这一研究问题进行了实验研究，给出了初步的敏感性结果。实验研究了对称泵浦的高Q硅棒谐振器在WGM衰减场中由于尖锐的钨尖端微扰而产生的腔响应。钨尖端到微谐振器的距离由纳米级精密压电台精确控制。报道的实验受到几个慢噪声源的影响，因此对尖端进行了调制以研究空腔响应。计算了位移谱密度，对于5s的持续时间测量，得到的最低噪声差为0.65nmHz1/2。信噪比均在20dB以上。所有测量的噪声下限似乎都遵循1/f的直线，这表明较高的调制频率可能会由于低噪声下限而提高灵敏度和信噪比。需要进一步的实验来充分确定钨尖端在谐振器逝去场中的主导效应。结果表明，这主要是由散射和折射率变化引起的。对确定每种效应的贡献的实验进行了概述。未来的工作还包括量化对称性破缺区域外的灵敏度，以与本文报告的灵敏度进行比较。

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.