Formation and radiation of unidirectional guided resonances in asymmetric gratings with simultaneously broken up-down mirror and in-plane <i>C</i>₂ symmetries

Unidirectional guided resonances (UGRs) in planar photonic lattices are distinctive resonant eigenstates that emit light in a single direction. A recent study has demonstrated that UGRs can be utilized to implement ultralow-loss grating couplers for integrated photonic applications. In this study, we investigate the formation and radiation of UGRs in two types of L-shaped gratings, type I and type II, which exhibit both broken up-down mirror symmetry and broken in-plane C 2 symmetry. In type I gratings, quasi-UGRs are readily identified in the lower band, whereas in type II gratings they appear in the upper band. We demonstrate that, as the relevant grating parameters are gradually varied, these quasi-UGRs evolve into genuine UGRs in the lower band for type I gratings and in the upper band for type II gratings. In type I gratings, UGRs produce negative-angle emission because their Poynting vectors are oriented antiparallel to their wavevectors, while in type II gratings, UGRs yield positive-angle emission due to the parallel alignment of their Poynting vectors and wavevectors. Moreover, the position and emission angle of UGRs can be systematically controlled by varying the lattice parameters. Our findings offer valuable insights for developing high-efficiency optical interconnects that leverage UGRs.