Width-independent and Robust Multimode Interference Waveguides Based on Anomalous Bulk States

Abstract: Multimode interference (MMI) is a fundamental physical principle that plays a crucial role in modern communication technologies for wave splitting, filtering, switching and multiplexing. Typically, the generation of multimodes is highly dependent on the waveguide's cross-section, particularly its width, by which the mode profiles and the interference patterns can be severely affected, leading to unstable MMI performance. Here, we realize width-independent and robust MMI waveguides. Our principle is based on the unique properties of multilayer graphene lattices. By properly modulating the boundary potential, this Dirac-type material supports anomalous bulk states with uniform wavefunctions independent of the sample size. Benefited from such an anomaly, the bulk states in a waveguide formed by multiple layers of graphene ribbons exhibit width-independent MMI. Enabled by this intriguing characterisitc, we construct 2*2 MMI waveguides using bilayer photonic and phononic crystals with graphene lattices. By precisely modulating their boundaries, the anomalous bulk states and correspondingly the width-independent MMI are achieved. Our experimental measurements show that the input wave energy travelling through the MMI waveguide can be split into two outputs with a frequency-tunable ratio, and due to the width-independent characteristic, the splitter is robust to geometric perturbations. We further demonstrate stable MMI across multiple interconnects with stepped widths, allowing for high power capacity while maintaining high coupling efficiency. Our approach successfully decouples MMI performance from waveguide width, creating lateral degrees of freedom that enable flexibly scalable and robust photonic, phononic, and electronic integrated circuits for versatile MMI applications.