Ultrahigh Q whispering gallery mode electro-optic resonators on a silicon photonic chip.

Crystalline whispering gallery mode (WGM) electro-optic resonators made of LiNbO₃ and LiTaO₃ are critical for a wide range of applications in nonlinear and quantum optics, as well as RF photonics, due to their remarkably ultrahigh Q(>10⁸) and large electro-optic coefficient. Achieving efficient coupling of these resonators to planar on-chip optical waveguides is essential for any high-yield and robust practical applications. However, it has been very challenging to demonstrate such coupling while preserving the ultrahigh Q properties of the resonators. Here, we show how the silicon photonic platform can overcome this long-standing challenge. Silicon waveguides with appropriate designs enable efficient and strong coupling to these WGM electro-optic resonators. We discuss various integration architectures of these resonators onto a silicon chip and experimentally demonstrate critical coupling of a planar Si waveguide and an ultrahigh QLiTaO₃ resonator (Q∼10⁸). Our results show a promising path for widespread and practical applications of these resonators on a silicon photonic platform.