Thin‐film lithium niobate (TFLN) has been widely used in electro‐optic modulators, acoustic‐–optic modulators, electro‐optic frequency combs, and nonlinear wavelength converters owing to the excellent optical properties of lithium niobate. The performance of these devices is highly dependent on the fabrication quality of TFLN. Although state‐of‐the‐art TFLN microrings with an intrinsic quality factor (Q‐factor) exceeding 1 × 107 have been realized by inductively coupled plasma–reactive ion etching (ICP‐RIE) and chemical mechanical polishing (CMP), ICP‐RIE has moderate throughput, moderate reproducibility, and high cost in etching TFLN, while CMP features moderate throughput and low cost in etching TFLN. Here, a wet etching method for TFLN, leading to the fabrication of a micro‐racetrack with an intrinsic Q‐factor of over 9.27 × 106 is developed. The suitability of this method to fabricate a narrow coupling gap between the bus waveguide and microring enables the coupling conditions of the microring to be customized. This method features a high throughput, a high reproducibility, and a low cost in etching TFLN, showing the potential to boost the mass production of integrated LN photonic devices with high fidelity and affordability dramatically.

Broadband mid-infrared frequency combs are of particular interest to mid-infrared spectroscopy due to their ruler-like precise discrete comb teeth. However, the state-of-the-art mid-infrared frequency combs are usually limited to low integration level and high pump power as a result of the conventional way of mid-infrared frequency comb generation—-producing a near-infrared frequency comb first and then convert it to mid-infrared regime through a nonlinear process. Here, we theoretically investigate two integrated designs for generating mid-infrared frequency combs with ultra-low power pump based on the lithium-niobate on insulator (LNOI) platform. Utilizing periodically poled lithium-niobate (PPLN) waveguides and microring electro-optic phase modulators, we switch the conventional order of comb generation and nonlinear conversion. This paradigm shift significantly improves the conversion efficiency of mid-infrared frequency comb generation and obviates the need for femtosecond lasers. Our theoretical results predict that a broadband mid-infrared frequency comb around 4.3 µm with nanowatt-power-level comb teeth can be produced from continuous-wave (CW) inputs whose power is lower than 5 mW with an ultra-high conversion efficiency above 1800 %/W. Our designs of mid-infrared frequency comb have high controllability, flexibility and integration level, enabling the miniaturization of mid-infrared spectrometers.

Lithium Niobate Microrings In article number 2208113, Rongjin Zhuang, Yang Li, and co‐workers develop a wet‐etching method for thin film lithium niobate (TFLN), leading to the fabrication of microrings with an intrinsic quality factor over 9 million. This method features high throughput, high reproducibility, and low cost in etching TFLN, showing the potential to boost the mass production of integrated TFLN devices with high fidelity and affordability dramatically.

Using wet etching method, we realized microrings with intrinsic quality factor over 1.19 × 107 based on thin film lithium niobate on insulator. This method features low cost, high throughput and high reproducibility.