Polarization-independent reciprocal lens based on non-Hermitian complex degenerate bands

Reciprocal lenses overcome key limitations of conventional imaging mechanism, which require precise optical-axis alignment and produce only inverted real images. This advancement highlights the potential of nonlocal metasurfaces for imaging applications. However, existing reciprocal lenses operate only under specific polarization conditions, hindering their practical utility. In this work, we propose a reciprocal lens based on a Stampfli-triangle photonic crystal slab. By achieving isotropy and degeneracy of the non-Hermitian complex bands of guided resonances within the operational in-plane wavevector domain, we effectively eliminate polarization dependence in the transmitted beam. The proposed lens operates for arbitrary polarization states and directly produces upright real images of complex objects. This design provides a new platform for beam shifting effects and wavevector domain optical signal manipulation, paving the way toward further miniaturization and integration of optical imaging systems that incorporate nonlocal flat devices.