Stable gigahertz- and mmWave-repetition-rate soliton microcombs on X-cut lithium niobate

Soliton microcombs are a cornerstone of integrated frequency comb technologies, with applications spanning photonic computing, ranging, microwave synthesis, optical communications, and quantum light generation. In nearly all such applications, electro-optic (EO) components play a critical role in generating, monitoring, stabilizing, and modulating the solitons. Towards building photonic integrated circuits for next-generation applications, that will simultaneously maximize system performance and minimize size, weight, and power consumption metrics, achieving soliton microcombs and efficient EO modulation on a chip is essential. X-cut thin-film lithium niobate (TFLN) has emerged as a leading photonic platform for the realization of high-performance integrated EO devices and systems. However, despite extensive research, soliton microcombs have remained elusive to X-cut TFLN due to its multiple strong Raman-active modes, in-plane refractive index anisotropy, and photorefractive effects. Here, we address this long-standing challenge and demonstrate versatile soliton microcombs on X-cut TFLN, with repetition-rates spanning from the gigahertz (~26 GHz) up to the millimeter-wave (~0.156 THz) regime. The combs feature exceptional long-term stability, maintaining a direct injection-locked state for over 90 minutes (manually terminated), with repetition-rate phase noise closely tracking that of a high-quality electronic microwave synthesizer. Our finding broadly advances both the fundamental science and practical applications of integrated comb sources by enabling efficient EO modulation and broadband coherent solitons to be monolithically combined on the same chip.