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1. Octave-spanning Kerr soliton frequency combs in dispersion- and dissipation-engineered lithium niobate microresonators

   https://doi.org/10.1038/s41377-024-01546-7  

   Song, Yunxiang; Hu, Yaowen; Zhu, Xinrui; Yang, Kiyoul; Lončar, Marko (December 2024, Light: Science & Applications)

   Abstract Dissipative Kerr solitons from optical microresonators, commonly referred to as soliton microcombs, have been developed for a broad range of applications, including precision measurement, optical frequency synthesis, and ultra-stable microwave and millimeter wave generation, all on a chip. An important goal for microcombs is self-referencing, which requires octave-spanning bandwidths to detect and stabilize the comb carrier envelope offset frequency. Further, detection and locking of the comb spacings are often achieved using frequency division by electro-optic modulation. The thin-film lithium niobate photonic platform, with its low loss, strong second- and third-order nonlinearities, as well as large Pockels effect, is ideally suited for these tasks. However, octave-spanning soliton microcombs are challenging to demonstrate on this platform, largely complicated by strong Raman effects hindering reliable fabrication of soliton devices. Here, we demonstrate entirely connected and octave-spanning soliton microcombs on thin-film lithium niobate. With appropriate control over microresonator free spectral range and dissipation spectrum, we show that soliton-inhibiting Raman effects are suppressed, and soliton devices are fabricated with near-unity yield. Our work offers an unambiguous method for soliton generation on strongly Raman-active materials. Further, it anticipates monolithically integrated, self-referenced frequency standards in conjunction with established technologies, such as periodically poled waveguides and electro-optic modulators, on thin-film lithium niobate. 

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2. High-speed tunable microwave-rate soliton microcomb

   https://doi.org/10.1038/s41467-023-39229-3  

   He, Yang; Lopez-Rios, Raymond; Javid, Usman A; Ling, Jingwei; Li, Mingxiao; Xue, Shixin; Vahala, Kerry; Lin, Qiang (December 2023, Nature Communications)

   Abstract Soliton microcombs are a promising new approach for photonic-based microwave signal synthesis. To date, however, the tuning rate has been limited in microcombs. Here, we demonstrate the first microwave-rate soliton microcomb whose repetition rate can be tuned at a high speed. By integrating an electro-optic modulation element into a lithium niobate comb microresonator, a modulation bandwidth up to 75 MHz and a continuous frequency modulation rate up to 5.0 × 10¹⁴Hz/s are achieved, several orders-of-magnitude faster than existing microcomb technology. The device offers a significant bandwidth of up to tens of gigahertz for locking the repetition rate to an external microwave reference, enabling both direct injection locking and feedback locking to the comb resonator itself without involving external modulation. These features are especially useful for disciplining an optical voltage-controlled oscillator to a long-term reference and the demonstrated fast repetition rate control is expected to have a profound impact on all applications of frequency combs. 

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3. High-efficiency and broadband Kerr comb generation in normal-dispersion x-cut lithium niobate microresonators

   https://doi.org/10.1126/sciadv.aeb5758  

   Song, Yunxiang; Li, Zongda; Zhu, Xinrui; Lippok, Norman; Erkintalo, Miro; Lončar, Marko (February 2026, Science Advances)

   The ability to generate efficient and coherent frequency combs using photonic integrated circuits offers tremendous potential for a range of applications. X-cut thin-film lithium niobate (TFLN) is promising for developing next-generation microcomb-driven photonic systems, providing a diversity of functionalities through combined ${\mathrm{\chi }}^{\left(3\right)}$and electro-optic effects. Normal-dispersion Kerr combs are critically needed because of their standout advantages, yet this dispersion regime remains unexplored on x-cut TFLN. Here, we design and demonstrate microresonators suitable for the robust generation of normal-dispersion Kerr combs. We show Kerr combs that substantially surpass state-of-the-art microcombs on x-cut TFLN in key performance metrics and an ultrabroad frequency comb whose existence is underpinned by both normal-dispersion Kerr dynamics and stimulated Raman scattering. Our work will unlock high-speed and low-energy-consumption photonic circuits for communications and signal processing enabled by a monolithic microcomb technology while also stimulating investigations of previously unknown nonlinear states that may synergize hybrid material nonlinearities. 

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4. Electrically empowered microcomb laser

   https://doi.org/10.1038/s41467-024-48544-2  

   Ling, Jingwei; Gao, Zhengdong; Xue, Shixin; Hu, Qili; Li, Mingxiao; Zhang, Kaibo; Javid, Usman A; Lopez-Rios, Raymond; Staffa, Jeremy; Lin, Qiang (December 2024, Nature Communications)

   Abstract Optical microcomb underpins a wide range of applications from communication, metrology, to sensing. Although extensively explored in recent years, challenges remain in key aspects of microcomb such as complex soliton initialization, low power efficiency, and limited comb reconfigurability. Here we present an on-chip microcomb laser to address these key challenges. Realized with integration between III and V gain chip and a thin-film lithium niobate (TFLN) photonic integrated circuit (PIC), the laser directly emits mode-locked microcomb on demand with robust turnkey operation inherently built in, with individual comb linewidth down to 600 Hz, whole-comb frequency tuning rate exceeding 2.4 × 10¹⁷ Hz/s, and 100% utilization of optical power fully contributing to comb generation. The demonstrated approach unifies architecture and operation simplicity, electro-optic reconfigurability, high-speed tunability, and multifunctional capability enabled by TFLN PIC, opening up a great avenue towards on-demand generation of mode-locked microcomb that is of great potential for broad applications. 

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5. Vernier microcombs for integrated optical atomic clocks

   https://doi.org/10.1038/s41566-025-01617-0  

   Wu, Kaiyi; O’Malley, Nathan P; Fatema, Saleha; Wang, Cong; Girardi, Marcello; Alshaykh, Mohammed S; Ye, Zhichao; Leaird, Daniel E; Qi, Minghao; Torres-Company, Victor; et al (April 2025, Nature Photonics)

   Abstract Kerr microcombs have drawn substantial interest as mass-manufacturable, compact alternatives to bulk frequency combs. This could enable the deployment of many comb-reliant applications previously confined to laboratories. Particularly enticing is the prospect of microcombs performing optical frequency division in compact optical atomic clocks. Unfortunately, it is difficult to meet the self-referencing requirement of microcombs in these systems owing to the approximately terahertz repetition rates typically required for octave-spanning comb generation. In addition, it is challenging to spectrally engineer a microcomb system to align a comb mode with an atomic clock transition with a sufficient signal-to-noise ratio. Here we adopt a Vernier dual-microcomb scheme for optical frequency division of a stabilized ultranarrow-linewidth continuous-wave laser at 871 nm to an ~235 MHz output frequency. This scheme enables shifting an ultrahigh-frequency (~100 GHz) carrier-envelope offset beat down to frequencies where detection is possible and simultaneously placing a comb line close to the 871 nm laser—tuned so that, if frequency doubled, it would fall close to the clock transition in¹⁷¹Yb⁺. Our dual-comb system can potentially combine with an integrated ion trap towards future chip-scale optical atomic clocks. 

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