Fully passive non-Hermitian skin effect from a waveguide-coupled photonic Su-Schrieffec-Heeger chain via loss-induced synthetic magnetic field

Unidirectional light transport in one dimension is of major interest in photonics, yet its realization has been fundamentally limited by the Lorentz reciprocity theorem. Existing nonreciprocal devices rely either on magneto-optic effects or spatiotemporal modulation, both of which often suffer from compatibility issues and device complexity. Recently, a novel transport phenomenon arising from open systems, namely the non-Hermitian skin effect, has attracted wide attention. While previous approaches for the one-dimensional skin effect utilized asymmetric coupling, such asymmetric coupling does not occur naturally in linearly coupled photonics, necessitating additional device engineering for its realization. Therefore, the practical implementation of the non-Hermitian skin effect requires a compact, magnet-free, and fully passive solution for optical nonreciprocity.

In this study, we propose a novel implementation scheme for optical nonreciprocity and the one-dimensional skin effect. Our model is based on a Su-Schrieffer-Heeger chain of photonic cavities coupled with auxiliary modes that introduce a pseudomagnetic field when loss is imposed. We show that the model can be implemented using photonic crystal nanobeam cavities. Numerical simulations of the device confirm clear nonreciprocity as well as the skin effect of photonic modes, validating our fully passive, magnet-free approach. Our study paves the way toward a magnet-free optical isolator and unidirectional light transport.