Unidirectional Guided Resonances Enabled by Competing Fourier Harmonics near the Fourth Stop Band

Unidirectional guided resonances (UGRs) have attracted considerable attention owing to their remarkable ability to radiate exclusively in one direction from single-layer planar photonic lattices without metallic components.

Conventionally, UGRs have been understood to require either broken in-plane $C_2$ symmetry or interband coupling between distinct modes.

Here, a new mechanism for realizing UGRs is presented, in which out-of-plane radiation near the fourth stop band is mediated by two distinct channels associated with the first and second Fourier harmonics.

When the radiation components from the first and second Fourier harmonics cancel each other out in both the upward and downward directions, nonradiative bound states in the continuum emerge.

By contrast, UGRs arise when such cancellation occurs only in one direction.

By tuning the lattice parameters, the positions of these UGRs can be controlled, allowing them to merge at the $\Gamma$ point.

The two-channel radiation-cancellation model enables UGRs without relying on in-plane symmetry breaking or interband coupling, thereby relaxing lithographic constraints and simplifying device design.

It also provides a useful framework for controlling topological singular states in higher-order photonic bands.