Ultra-high-Q Microcavities for D$_{2}$O Detection

Ultra high Q optical microcavities (Q $>$ 10$^{6})$ provide a method for boosting detection sensitivity over conventional detection methods. In a recently published work, the difference between quality factors (Q) of microcavities immersed in D$_{2}$O and H$_{2}$O was measured and calculated. Due to the higher absorption of H$_{2}$O, resonators immersed in H$_{2}$O have lower Q's than those immersed in D$_{2}$O. This difference in absorption can be exploited to use the resonator as a D$_{2}$O detector. The effect on Q is most noticeable at 1300nm, where the Q in water is 10$^{6}$ and the Q in D$_{2}$O is 10$^{7}$. At longer wavelengths, both Q(D$_{2}$O) and Q(H$_{2}$O) are further degraded, reducing the efficacy of detection, and at shorter wavelengths, Q(D$_{2}$O) and Q(H$_{2}$O) become similar, reducing the effect that the presence of D$_{2}$O alone has on Q. By monitoring the quality factor, .01 M concentration of D$_{2}$O in water (10$^{17}$ molecules of D$_{2}$O) has been detected; however, the actual sensitivity limit is far better than this concentration value. Detection has also been demonstrated by cyclic introduction and flushing of D$_{2}$O, leading to cyclic degradation and improvement of the Q, thereby demonstrating reversible detection.