Exceptional point induced robust unidirectional guided resonance in perturbed silicon nanogratings.

Unidirectional guided resonances (UGRs) enable static unidirectional radiation of eigenstates and have been widely studied. Exceptional points (EPs) in non-Hermitian open systems provide a unique perspective for exploring UGRs. However, existing studies mainly focus on the topological properties of UGRs, while how the region delimited by an EP affects the radiation behavior of UGRs remains unclear. Here, we investigate the radiation of EP-related UGRs in a notch-perturbed silicon nanograting. At notch parameters Δ = 0.3 and δ = 0.045, the complex eigenfrequencies of two modes coalesce to form an EP. In the region delimited by the EP, where the imaginary parts remain nearly degenerate, a UGR with an asymmetric radiation ratio η of 41 dB appears. The radiative losses of the modes remain close in this region, allowing η to remain high under structural perturbations of the grating and variations in the ambient refractive index n2. Further tuning of the notch parameters shows that changes in interband coupling drive the UGR to migrate on the same band toward the critical interband coupling state associated with the EP. These results indicate that the region delimited by the EP improves the geometrical tolerance of UGRs and provides a route to designing devices with robust unidirectional radiation.