Beam Splitter With Arbitrary Splitting Ratio by Valley Edge Mode

ABSTRACT Photonic crystal beam splitters based on coupled topological interface states (TISs) circumvent the long coupling distances of conventional waveguides. However, their splitting ratio is quantized, limited by the discrete coupling lengths imposed by the lattice period, and their operation bandwidth is narrow due to the phase‐matching requirement. To overcome these limitations, we propose and experimentally demonstrate a topological beam splitter composed of three intersecting interface channels coupled through a heterostructure. By engineering the symmetry‐breaking parameters of the heterostructure, we achieve continuous control over the TIS mode profiles and the inter‐channel coupling strength. This enables continuous tuning of the beam‐splitting ratio, defined as the normalized field distribution between the two output ports, from 0:100 to 100:0. Crucially, the device bandwidth is determined by the intrinsic bandwidth of the TISs, which is significantly broader than that of phase‐matched coupled waveguides. Our design provides a versatile platform for broadband, arbitrarily tunable beam splitting, promising advancements in wavelength‐division multiplexing, on‐chip optical communications, and topological lasers.