Circularly Polarized Polariton Lasing from Spin-Momentum Locking in Deformed Plasmonic Kagome Cavities

This paper describes room-temperature polariton lasing with high circular polarization from plasmonic Kagome lattice cavities strongly coupled to colloidal semiconducting quantum wells. By shrinking and expanding the trimer unit cells in Al nanoparticle lattices, the inversion symmetry is broken, resulting in the spin-momentum locking of cavity photons. Circularly polarized lattice resonances emerged from crossings of different diffraction orders, denoted as T points, along the Γ-K direction in momentum space and whose degree of circular polarization is higher than those of high-symmetry K-points. Deformed Kagome lattice cavities combined with CdSe nanoplatelets enabled the formation of exciton-polaritons. Spin-selectivity from cavity modes resulted in control over the handedness of circular polarization as well as direction of photoluminescence from lower polaritons. Six lasing beams from T points has alternating handedness, low thresholds (8 µJ cm^{- 2}), and high degrees of circular polarization (≈0.7). The anticipation that spin-momentum locking via plasmonic Kagome lattices can be extended to other non-Bravais metasurfaces with hexagonal symmetries and opens prospects for exploring optical analogues of spin and valley physics.