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 H 2 O and D 2 O, and results are then compared to predictions of a numerical model. Quality factors ͑Q͒ in excess of 10 8 , a factor of 100 higher than previous measurements in an aqueous environment, are observed in both High-Q and ultra-high-Q ͑UHQ͒ silica optical microcavities can perform as highly sensitive detectors; 1-3 they derive their excellent transduction abilities from long photon lifetimes within the whispering gallery of the microcavity. Unlike their optical waveguide counterparts, wherein the photon interacts with a functionalized surface only once, 4,5 recirculation within the microcavity allows photons to interact with the surface many times. Additionally, the surface of silica-based microcavities is easily sensitized using silanization agents, 6 amines, carbohydrates, the biotin-streptavidin system, 7 or antibodies. 8 For example, silica microsphere resonators, with a properly sensitized surface, were recently used to distinguish between two strands of DNA. 9 However, while detailed studies have been performed to determine the limits of a resonator's quality factor in air, 10-12 no comparable studies have been performed in water. Because most detection experiments are performed in a water-based solution, it is important to thoroughly understand the impact of this environment on the relationship between the diameter of the microtoroid resonator and the operational wavelengths of interest.To this end, UHQ silica microtoroid resonators were fabricated over a wide range ͑50-250 m͒ of major toroid diameters as shown in Fig. 1. 13 Experiments were performed in both water ͑H 2 O͒ and deuterium oxide ͑D 2 O͒. The D 2 O was purchased from Aldrich. D 2 O was chosen as the second liquid because it has the same refractive index and, in turn, the same radiation loss as H 2 O. However, its absorption at all wavelengths tested is significantly less. 14 This allowed for selective probing of the absorption-loss mechanism and verification of the model developed to describe this system.The model used finite element analysis to predict the Q-factor of microtoroid resonators immersed in water or D 2 S and accounted for two loss mechanisms: radiation loss and absorption loss. The absorption loss was numerically calculated at ͑680, 1300, 1570͒ nm using published values of H 2 O ͑0.0045, 1.12, 8.79͒ cm −1 and D 2 O ͑2.46 ϫ 10 −4 , 0.105, 0.327͒ cm −1 . 14 The modeling of radiation loss uses a fully vectorial two-dimensio...
