Janus bound states in the continuum with complex asymmetric topological charges

Janus bound states in the continuum (BICs) have emerged as a promising platform for manipulating phase singularities due to their asymmetric topological charges in upward and downward radiations. However, current realizations of Janus BICs are limited to the configuration with a topological charge of +1 for upward radiation and −1 for downward radiation, with asymmetric topological states where q≠±1 remaining unachieved. To address this limitation, we propose a novel solution to realize Janus BICs with complex asymmetric topological charges. The designed structure comprises two metasurfaces of differing thicknesses, separated by an air gap. A perturbation is applied only to the upper metasurface to break the C6 symmetry while preserving the C3 symmetry. This strategy causes the originally symmetry-protected BIC with a topological charge of −2 at the Γ point to split into six C points on one side, each carrying a topological charge of −1/2, thereby changing the overall topological charge from −2 to +1. On the opposite side, due to the presence of the middle air layer and the breaking of σz symmetry, the six C points merge at the Γ point, ultimately achieving a Janus BIC with a topological charge of +1 for upward radiation and −2 for downward radiation. By further adjusting the perturbation, the topological charges on the upper and lower radiations can be interchanged. This study not only enriches the variety of Janus BICs but also provides new avenues for realizing the coexistence of high-order and first-order vortex beams in subsequent applications.