  Thin-film lithium niobate (TFLN) is superior for integrated nanophotonics due
to its outstanding properties in nearly all aspects: strong second-order
nonlinearity, fast and efficient electro-optic effects, wide transparency
window, and little two photon absorption and free carrier scattering. Together,
they permit highly integrated nanophotonic circuits capable of complex photonic
processing by incorporating disparate elements on the same chip. Yet, there has
to be a demonstration that synergizes those superior properties for system
advantage. Here we demonstrate such a chip that capitalizes on TFLNs favorable
ferroelectricity, high second-order nonlinearity, and strong electro-optic
effects. It consists of a monolithic circuit integrating a Z-cut, quasi-phase
matched microring with high quality factor and a phase modulator used in active
feedback control. By Pound-Drever-Hall locking, it realizes stable frequency
doubling at about 50% conversion with only milliwatt pump, marking the highest
by far among all nanophotonic platforms with milliwatt pumping. Our
demonstration addresses a long-outstanding challenge facing cavity-based
optical processing, including frequency conversion, frequency comb generation,
and all-optical switching, whose stable performance is hindered by
photorefractive or thermal effects. Our results further establish TFLN as an
excellent material capable of optical multitasking, as desirable to build
multi-functional chip devices.
