Soliton microcomb generation at 2  μm in z-cut lithium niobate microring resonators

Chip-based soliton frequency combs have been demonstrated on various material platforms, offering broadband, mutually coherent, and equally spaced frequency lines desired for many applications. Lithium niobate (LN), possessing both second- and third-order optical nonlinearities, as well as integrability on insulating substrates, has emerged as a novel source for microcomb generation and controlling. Here we demonstrate mode-locked soliton microcombs generated around 2 μm in a high-Q z-cut LN microring resonator. The intracavity photorefractive effect is found to be still dominant over the thermal effect in the 2 μm region, which facilitates direct accessing soliton states in the red-detuned regime, as reported in the telecom band. We also find that intracavity stimulated Raman scattering is greatly suppressed when moving the pump wavelength from the telecom band to 2 μm, thus alleviating Raman-Kerr comb competition. This Letter expands mode-locked LN microcombs to 2 μm, and could enable a variety of potential applications based on LN nanophotonic platform.