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On‐chip optical microresonators are essential building blocks in integrated optics. The ability to arbitrarily engineer their resonant frequencies is crucial for exploring novel physics in synthetic frequency dimensions and practical applications like nonlinear optical parametric processes and dispersion‐engineered frequency comb generation. Photonic crystal ring (PhCR) resonators are a versatile tool for such arbitrary frequency engineering, by controllably creating mode splitting at selected resonances. To date, these PhCRs have mostly been demonstrated in isotropic photonic materials, while such engineering can be significantly more complicated in anisotropic platforms that often offer more fruitful optical properties. Here, the spectral engineering of chip‐scale optical microresonators is realized in the anisotropic lithium niobate (LN) crystal by a gradient design that precisely compensates for variations in both refractive index and perturbation strength. Controllable frequency splitting is experimentally demonstrated at single and multiple selected resonances in LN PhCR resonators with different sizes, while maintaining high quality‐factors up to 1 × 10 6 . Moreover, a sharp boundary is experimentally constructed in the synthetic frequency dimension based on an actively modulated x‐cut LN gradient‐PhCR, opening up new paths toward the arbitrary control of electro‐optic comb spectral shapes and exploration of novel physics in the frequency degree of freedom.

Spectral Engineering of Optical Microresonators in Anisotropic Lithium Niobate Crystal