Multiple unidirectional guided resonances with reversible radiation direction in tetramer metagratings

Unidirectional guided resonances (UGRs), as a characteristic class of polarization singularities in momentum space, are emerging as key enablers in advanced nanophotonic devices for manipulating radiation asymmetry. Here, we theoretically propose a refractive-index-perturbation strategy, enabled by the excellent electro-optic Pockels effect of lithium niobate, to realize multiple UGRs in a tetramer metagrating. Numerical simulations reveal that a tetramer metagrating simultaneously preserving C2 and mirror symmetries supports two symmetry-protected bound states in the continuum (BICs). When the refractive indices of two nanostrips along one diagonal of the tetramer metagrating are adjusted simultaneously, the structure loses its mirror symmetry but remains in C2 symmetry. As a result, each BIC splits along the kx axis into a pair of UGRs that share identical evolution characteristics yet radiate in completely opposite directions. Ultimately, four outstanding UGRs are obtained, each exhibiting Q-factors exceeding 105 and radiation asymmetry ratios approaching unity. The underlying physics for these UGRs is the migration of polarization vortex singularities—initially coincident at Γ point for both top and bottom ports—which split and drift in the opposite directions along the kx axis upon breaking the mirror symmetry. Interestingly, switching the refractive-index perturbation between the primary and secondary diagonals reverses the radiation direction of all four UGRs. Moreover, applying the refractive-index perturbation to only one nanostrip in the tetramer allows for simultaneous control over both the evolutionary pathways and the radiation directions of the UGRs. Our findings establish a refractive-index-asymmetric scheme to enrich the fundamental understanding of UGR physics.