Acousto-Optic Nonreciprocal Polarization Rotation on X-Cut Thin-Film Lithium Niobate

Polarization-dependent nonreciprocal behavior of integrated optical isolators plays a significant role in the functionality of photonic integrated circuits. However, conventional magneto-optic (MO) isolators based on the Faraday effect suffer from high propagation loss and fabrication complexity. Here, we demonstrate an integrated magnetic-free nonreciprocal polarization rotation device based on acousto-optic (AO) scattering, implemented on an X-cut thin-film lithium niobate (TFLN) platform. Unlike magneto- or electro-optic configurations, our device employs electrically driven surface acoustic waves to facilitate energy and momentum conversion between fundamental polarization mode pairs. By leveraging the inherent anisotropy of TFLN, efficient interpolarization conversion and large single-sideband suppression are achieved. Furthermore, a folded four-port configuration is designed to extend the AO interaction length, thereby demonstrating nonreciprocal optical propagation. The device achieves a high nonreciprocal contrast exceeding 20 dB and demonstrates a broad operational bandwidth of more than 150 GHz within the optical C band. Additionally, the optimized device exhibits a figure of merit (FoM) greater than 8 rad/dB, approximately 1 order of magnitude higher than that of state-of-the-art on-chip MO devices. These findings provide a promising route toward developing broadband nonmagnetic integrated optical isolators and enabling efficient polarization rotation or frequency conversion on other anisotropic material platforms.