Ultra-high-$Q$ microcavity operation in $H2O$ and $D2O$

Optical microcavities provide a possible method for boosting the detection sensitivity of biomolecules. Silica-based microcavities are important because they are readily functionalized, which enables unlabeled detection. While silica resonators have been characterized in air, nearly all molecular detections are performed in solution. Therefore, it is important to determine their performance limits in an aqueous environment. In this letter, planar microtoroid resonators are used to measure the relationship between quality factor and toroid diameter at wavelengths ranging from visible to near-IR in both $H2O$ and $D2O$⁠, and results are then compared to predictions of a numerical model. Quality factors $(Q)$ in excess of $108$⁠, a factor of 100 higher than previous measurements in an aqueous environment, are observed in both $H2O$ and $D2O$⁠.