Multipolar perspective on unidirectional guided resonances

The recently reported unidirectional guided resonances (UGRs) in asymmetric photonic crystal (PhC) slabs exhibit ultrahigh unidirectional radiation. Radiation on one side of the PhC slab at a particular direction is allowed while that on the other side is perfectly canceled. The UGR was previously explained as the single side far field polarization singularity. Here the formation mechanism of UGRs is explained from the viewpoint of multipole radiation. Two types of UGR are considered with one induced from the bound state in the continuum and the other arising from the avoided crossings of two guided resonances. The first type of UGR, similar to the Brewster effect, arises from the overlap of the null direction of the dipolelike radiation with the system radiation channel. The second type of UGR stems from the intermixing of multipoles in the two original bands which modifies the radiation singularities, resulting in the single side radiation in one of the newly formed bands. Our results provide an alternative perspective for the understanding of UGRs by connecting the near field information and the far field radiation.