Broadband, robust, and tunable beam splitter based on topological unidirectional surface magnetoplasmons

Beam splitters are key components in integrated microwave and photonic systems. However, conventional designs based on directional coupling or multimode interference often suffer from backscattering, frequency-dependent splitting ratios, and limited bandwidth. To overcome these limitations, we propose a new physical mechanism here to achieve a broadband, robust, and tunable beam splitter by manipulating the mode coupling of topological unidirectional surface magnetoplasmons (USMPs) at the input and output waveguides. We demonstrate that the beam splitter not only exhibits strong robustness against obstacles but also achieves a broad bandwidth across nearly the entire USMP band, with arbitrarily tunable and frequency-independent splitting ratios. Moreover, the operating band of the beam splitter can be actively tuned by adjusting the external magnetic field while retaining its robust and broadband characteristics. Our results extend the research frontier of beam splitters and may have potential applications in integrated photonic devices and modern communication systems.