Spin-momentum locking breakdown on plasmonic metasurfaces

We present a scattering formalism to analyze the spin-momentum locking in structured holey plasmonic metasurfaces. It is valid for any unit cell for arbitrary position and orientation of the holes. The spin-momentum locking emergence is found to originate from the unit-cell configuration. Additionally, we find that there are several breakdown terms spoiling the perfect spin-momentum locking polarization. We prove that this breakdown also appears in systems with global symmetries of translation and rotation of the whole lattice, like the kagome lattice. Finally, we present the excitation of surface-plasmon polaritons as the paramount example of the spin-momentum locking breakdown.