Reconfigurable generation of optical and spatiotemporal vortices based on VO₂ phase transition regulated terahertz metasurfaces

Optical vortices (OVs) carry longitudinal orbital angular momentum (OAM), while spatiotemporal optical vortices (STOVs) possess transverse OAM. The integration of these functionalities presents challenges due to their distinct physical mechanisms. In this study, we propose a terahertz multifunctional reflective device that utilizes vanadium dioxide (VO₂) phase-change films in conjunction with a metasurface, enabling controllable switching between OV and STOV generation. When VO₂ is in the insulating state, the device provides spiral phase reflection for circularly polarized incident light, generating topological OVs. Conversely, when VO₂ transitions to its metallic phase, the reflection leads to the formation of topological dark spots corresponding to STOVs. We observe the evolution of both singularities under varying parameters. An analysis of the structural parameters and VO₂ loss reveals that both singularities exhibit topological robustness in wave vector space. Reversible switching is achieved through dynamic temperature control. Our terahertz device, integrating two vortex beams, offers unified multi-dimensional OAM regulation and opens pathways for vortex light applications in integrated information processing and terahertz communication.