Arbitrary Reflectionless Optical Routing via Non-Hermitian Zero-Index Networks

Abstract: Optical routers are fundamental to photonic systems, but their performance is often limited by unwanted reflections and constrained functionalities. Existing design strategies generally lack complete control over reflectionless pathways and typically require computationally intensive iterative optimization. A general analytical framework for the inverse design of arbitrary reflectionless routing has remained unavailable. Here, we present an analytical inverse-design approach based on non-Hermitian zero-index networks, which enables arbitrary reflectionless routing for nearly any desired scattering response. By establishing a direct algebraic mapping between target scattering responses and the network's physical parameters, we transform the design process from iterative optimization into deterministic calculation. This approach enables the precise engineering of arbitrary reflectionless optical routing. We demonstrate its broad utility by designing devices from unicast and multicast routers with full amplitude and phase control to coherent beam combiners and spatial mode demultiplexers in four-port and six-port networks. Our work provides a systematic and analytical route to designing advanced light-control devices.