Resonantly Enhanced Tunable Backward Brillouin-Induced Transparency with Intrinsic Nonreciprocity in Thin-Film Lithium Niobate Microring

Efficient excitation and control of backward stimulated Brillouin scattering (BSBS) are essential for nonreciprocal photonics and microwave signal processing, yet achieving tunable Brillouin scattering-induced transparency (BSIT) with built-in nonreciprocity in an integrated platform remains challenging. In this study, a cavity-enhanced backward BSIT architecture is experimentally demonstrated on a thin-film lithium niobate microring resonator (TFLN-MRR), where the free spectral range (FSR) of 8.9 GHz is designed to approximate the Brillouin shift. By injecting counter-propagating pump and probe lights with a wavelength difference matching the Brillouin shift, resonantly enhanced BSBS is achieved, with the gain peak exhibiting a 3 dB line width of 9.3 MHz. Two-dimensional tunability of the BSIT window is demonstrated: the BSBS gain rises from 1.06 to 14.81 dB with an estimated onset power of 20.4 mW, and the center frequency is continuously tuned over 374.2 MHz when tuning the pump wavelength. Leveraging the directionality of the wavevector, the device exhibits nonreciprocal transmission with an output power difference exceeding 20 dB between counterclockwise and clockwise pump injection, which remains stable over 30 min. These results establish the TFLN-MRR as a versatile platform supporting efficient BSBS, widely tunable BSIT, and robust nonreciprocal transmission for integrated photonic circuits.