Magnetic-Free Optical Mode Degeneracy Lifting in Lithium Niobate Microring Resonators

Breaking time-reversal symmetry in integrated photonics without magnetic fields remains a fundamental challenge. We demonstrate phonon-induced nonreciprocity through direct lifting of forward-backward mode degeneracy in microring resonators. Coherent acousto-optic coupling generates differential AC Stark shifts between counterpropagating fundamental optical modes, eliminating the need for intermodal conversion or complex photonic structures. Simple microwave excitation of integrated piezoelectric transducers provides dynamic control of nonreciprocal response, with experimentally demonstrated mode splitting exceeding twice the optical linewidth. The linear relationship between the splitting and acoustic power enables real-time reconfigurability across a wide range of optical wavelengths. This mechanism requires only simple microring resonators and fundamental optical modes, transforming nonreciprocity from a specialized technique requiring careful modal engineering to a universal, electrically controlled functionality. Our approach establishes a new paradigm for magnetic-free optical isolation and dynamic topological photonics.