Nonreciprocal and High‐Efficiency Monolithic Intermodal Conversion Driven by Surface Acoustic Waves on Thin‐Film Lithium Niobate

ABSTRACT Acousto‐optic (AO) modulation on integrated photonic platforms enables dynamic manipulation of light, with optical nonreciprocity serving as a key functionality for robust system operation. However, achieving high‐efficiency AO nonreciprocity typically requires the heterogeneous integration of piezoelectric materials, which introduces acoustic discontinuities and limits scalable integration. Here, we demonstrate monolithic thin‐film lithium niobate (TFLN) acousto‐optic modulators that realize efficient intermodal conversion and strong optical nonreciprocity. By incorporating a compact unidirectional spiral waveguide together with reflector‐enhanced surface acoustic waves, the device achieves a maximum intermodal conversion efficiency of −6.65 dB and a peak nonreciprocity contrast of 25.36 dB, while maintaining a nonreciprocal contrast exceeding 10 dB over a 0.6‐nm wavelength window. To further improve acoustic confinement, we introduce a suspended architecture that enhances AO coupling and enables −17.4 dB modulation efficiency at a significantly reduced RF power. In this configuration, a peak nonreciprocal contrast of 27.39 dB is achieved together with over 10 dB nonreciprocal bandwidth of 0.9 nm. These results highlight TFLN as a powerful platform for scalable acousto‐optic integration. The demonstrated AO nonreciprocity establishes a key building block for next‐generation lithium‐niobate photonic systems, paving the way toward compact, multifunctional signal processing, system stabilization, and light–matter interactions on TFLN platform.