Chiral Lasing via Broken Parity-Time Symmetry in Bound-State-in-the-Continuum Metasurfaces

Abstract We propose a concept for chiral lasing from planar metasurfaces that obviates the need for traditional out-of-plane symmetry breaking by exploiting spatial gain–loss modulation to break parity-time symmetry. We explain the underlying non-Hermitian physics of this design principle using a coupled-mode model of a four-site plaquette. The symmetry requirements for such chiral emission are explained with a general symmetry analysis based on projection operator matrices, which is implemented algorithmically for automated evaluation. This method enables the design of planar metasurfaces capable of emitting nearly pure circularly polarized light. We apply our analysis to simulations of both symmetric and asymmetric versions of a Fylfot metasurface design and demonstrate that the gain mode at the parity-time-symmetric exceptional point exhibits chiral emission. Lastly, we present a readily manufacturable metasurface made from an InGaAs slab, showing that such a metasurface laser can be actively tuned from linear to circular polarizations.