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

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 [Formula: see text] 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.