Billion Q-factor in silicon WGM resonators

Optical whispering gallery mode (WGM) resonators allow combination of small mode volume with high $Q$-factor. Silicon is a major material for modern microelectronics and photonics. However, relatively low $Q$-factors of optical Si microresonators demonstrated so far have limited some promising applications. We report what we believe is first time measurement of a $Q$-factor of $1.2\times {10}^9$ in millimeter scale crystalline silicon optical resonators at 1550 nm wavelength. A novel silicon hemispherical coupler allowed us to reach up to 35% of coupling efficiency.