Cascaded phase-change metasurfaces for orbital angular momentum mode switching

Optical orbital angular momentum (OAM) mode switching is vital for advancing signal modulation and channel multiplexing. Herein, a controllable spiral phase modulation strategy using cascaded phase-change metasurfaces is presented. By constructing Ge₂Sb₂Te₅ (GST) meta-atom arrays via the Pancharatnam-Berry phase theory, the conjugate spiral phase gradients are imparted to orthogonal circular polarization components. Utilizing the high phase response contrast between GST’s amorphous and crystalline states, we achieve switchable spiral phase patterns, enabling controllable OAM mode switching at a sub-nanosecond timescale. The paired GST metasurfaces can continuously switch seven OAM modes (from −3 to +3) with purities exceeding 81.3 %. Consequently, we demonstrate high-fidelity OAM-encoded shift-keying modulation transmission of four gray images with a decoding accuracy above 99.82 %, and transmit 16-QAM signals in an eight-channel OAM mode multiplexing communication system with bit-error-rates near 1 × 10⁻⁵. Our findings offer a scalable and compact solution for OAM mode switching, advancing OAM-based optical communication systems and networks.