Multistate and dispersion-controlled symmetric/asymmetric photonic spin-orbit interactions via Ge₂Sb₂Te₅ phase-change...

Metasurfaces enable flexible manipulation of the polarization, phase, and amplitude of light by engineering subwavelength structures, providing a pivotal platform for exploring photonic spin-orbit interactions (PSOIs). Nevertheless, it commonly suffers from limited design degrees of freedom, i.e., supporting either symmetric PSOIs (SPSOIs) or asymmetric PSOIs (APSOIs) independently for one metasurface. Herein, we propose a reconfigurable phase-change metasurface with Ge₂Sb₂Te₅ (GST) that achieves multistate and dispersion-switchable SPSOI/APSOI dual-mode manipulation. By tuning the 3 crystallization fractions of GST (i.e., amorphous, semicrystalline, and crystalline), such a metasurface can co-generate or switch between SPSOIs and APSOIs at 4 wavelengths. As conceptual demonstrations, a dual-channel orbital angular momentum (OAM) vortex beam generator and a dynamic hologram metadevice are designed and characterized, which verifies that the superposition of OAM states can be modulated via wavevector engineering by adjusting GST crystallization states, operating wavelength, and incident circular polarization. This proposed phase-change metasurface architecture provides a versatile route toward reconfigurable dynamic holographic displays, high-dimensional high-capacity optical information storage, and integrated multifunctional photonic devices.