Width‐Independent and Robust Multimode Interference Waveguides Based on Anomalous Bulk States

Abstract Multimode interference (MMI) underpins critical functionalities in wave splitting, filtering, switching, and multiplexing. Conventional MMI waveguides, however, suffer from instability due to their high dependence on waveguide dimensions, particularly the width. Here, width‐independent and robust MMI waveguides using graphene‐inspired Dirac metamaterials are realized. Leveraging lattice symmetry, these materials support anomalous bulk states with uniform wavefunctions through boundary modulation, independent of sample size and robust against material perturbations. Stacking multiple layers of such materials in a waveguide generates vertical MMI that inherits width independence and defect immunity. Experimentally, a 2 × 2 MMI acoustic power splitter is demonstrated. Self‐images of an input field are observed at periodic intervals along the propagation direction, with the output power split at an arbitrary splitting ratio controlled by frequency. Remarkably, the MMI maintains stability across multiple waveguides with stepped widths, exhibiting high‐coupling efficiency. Extending to optics, a silicon‑based design is proposed, and photonic anomalous bulk states and width‐independent, stable optical MMI at telecom wavelength (≈1550 nm) are numerically validated. This approach decouples multimode‐interference from waveguide dimensions, redistributing power in the vertical direction while freeing the lateral dimension, which is advantageous for compact, high‐efficiency, and fabrication‐tolerant MMI devices.