Refractometry with Ultralow Detection Limit Using Anisotropic Whispering-Gallery-Mode Resonators

The intrinsic sensitivity of whispering-gallery-mode resonators aimed at measuring refractive index can be extremely high, although their practical performance is compromised by temperature fluctuations that masquerade as refractive-index changes. We present a triple-mode approach that delivers simultaneous and independent sensing of temperature and refractive-index changes in the same resonator. The frequency difference between two orthogonally polarized modes is used to sense temperature which is then actively stabilized to similar to 1 mu K over 15 minutes. We then detect a frequency difference between two modes of different wavelengths to obtain a refractive-index measurement that is free of temperature fluctuations. This triple-mode technique delivers a state-of-the-art detection limit of 8 x 10(-9) refractive-index units, despite the resonator size being 100 times larger than that typically used for sensitive refractometric sensing.