Demonstration of ultra-high-Q small mode volume toroid microcavities on a chip

Optical microcavities confine light spatially and temporally and find application in a wide range of fundamental and applied studies. In many areas, the microcavity figure of merit is not only determined by photon lifetime (or the equivalent quality-factor, $\mathrm{Q}$ ), but also by simultaneous achievement of small mode volume $\left(\mathrm{V}\right)$ . Here we demonstrate ultra-high $\mathrm{Q}$ -factor small mode volume toroid microcavities on-a-chip, which exhibit a Q∕V factor of more than ${10}^6{(\lambda ∕n)}^{-3}$ . These values are the highest reported to date for any chip-based microcavity. A corresponding Purcell factor in excess of 200 000 and a cavity finesse of $>2.8\times {10}^6$ is achieved, demonstrating that toroid microcavities are promising candidates for studies of the Purcell effect, cavity QED or biochemical sensing.