Loss-induced Janus bound states in the continuum and topological charge reversals

Bound states in the continuum (BICs) provide a powerful platform for controlling topological radiation. Currently, multiple topological charge reversals (TCRs) of BICs have not yet been widely investigated. Here, we propose a loss-engineering approach to dynamically realize two instances of TCRs of BICs mediated by a Janus BIC. Introducing tunable loss in one layer breaks the out-of-plane mirror symmetry while preserving the in-plane symmetry, resulting in asymmetric evolution of polarization singularities in the two radiation channels. At a critical loss, the circularly polarized states (C points) in the downward channel merge at the Γ point, forming a Janus BIC with asymmetric topological charges. Further increasing the loss regenerates C points with opposite charges, inducing a charge reversal of the BIC. During this loss-driven transition, the topological charges of the (upward, downward) channels evolve from (+1, +1) to (+1, -1) and finally to (-1, -1), exhibiting two instances of TCRs. These results highlight loss engineering as a powerful tool for manipulating BIC topology in chiral and non-Hermitian photonic systems.