Reconfigurable mid-infrared metasurfaces with loss-controlled achromatic spin–orbit state detection

Simultaneous manipulation of the spin (polarization) and orbital angular momentum (OAM) of light in the mid-infrared is fundamentally constrained by chromatic dispersion and the static nature of geometric phase profiles in conventional flat optics. Here, we demonstrate that a reconfigurable Ge2Sb2Se4Te1 metasurface, reversibly switchable between amorphous and crystalline states, serves as a reconfigurable platform in which the tunable loss contrast actively controls the spin–orbit mapping. The phase transition not only modulates the transmission amplitude but fundamentally alters the local magnetic resonance symmetry, giving rise to a switchable mapping between the incident spin states and the focal-plane OAM distributions. By engineering the dispersion invariance of the Pancharatnam–Berry phase across the 3–3.3 μm bandwidth, the system achieves an achromatic topological response in which the topological charge number remains robust against wavelength variations. Our results reveal a strictly linear topological charge detection from l = −6 to +6, which rigorously satisfies the sampling constraint imposed by the discrete meta-atom lattice, while quantitative Stokes analysis yields a high spin–orbit mapping fidelity with relative errors below 9% under various incident orbital angular momentum (OAM) superpositions. These findings establish that the simultaneous detection of OAM and polarization directly reflects the underlying spin–orbit mapping enabled by the Pancharatnam–Berry phase in discretized flat optics, and they provide a reconfigurable platform for exploring multi-dimensional photonic state detection in the broadband mid-infrared regime.