Light confinement in a low-refraction-index microcavity bonded on a silicon substrate

The trapping of photons in low-refraction-index materials is thought to be prohibited in conventional photonic structures that employ total internal reflection. Specifically, the whispering gallery mode (WGM) microcavity, which is an important optical component, has to rely on a high contrast of the refraction index with the surrounding environment to manifest excellent light confinement. Here, we propose and demonstrate experimentally an optical microcavity structure consisting of a low-refraction-index silica microtoroid directly bonded on a high-refraction-index silicon substrate. The resonant structure supports high-Q fundamental WGMs in both visible and communication bands, while higher-order modes are suppressed significantly due to the strong leakage into the silicon substrate at long wavelengths. The highest measured quality factor exceeds ten million and low-threshold microcavity Raman lasing is also realized.