Tunable terahertz focused vector vortex beam generation enabled by cascaded metasurfaces

Focused vector vortex beams (VVBs) offer significant potential for applications in nonlinear effects, quantum optics, and communications due to their symmetric intensity patterns, phase singularities, and structured polarization profiles. Nevertheless, the emerging frontier of dynamically tunable VVBs in the THz regime faces critical limitations in conventional static metasurface implementations, hindering their full potential for advanced photonic applications. In this work, we propose and demonstrate a design strategy, which employs dielectric cascaded metasurfaces to generate VVBs with tunable characteristics through mechanical twisting. To achieve this, Layer I is constructed from birefringent silicon pillars with rectangular configurations, enabling independent encoding of orthogonal circularly polarized channels via spin-decoupled phasing techniques, while Layer II is composed of cylindrical silicon pillars with polarization-maintaining properties to control the focal length. The generation and modulation of VVBs are achieved by mechanically adjusting the relative angles between these two layers, allowing for dynamic tuning of the beam's properties. Experimentally, we further present the accurate generation of first- and second-order focused VVBs with a high focusing efficiency (> 12.9%), consistent with theoretical predictions. Moreover, the system exhibited continuous focal length tuning across 26 lambda-10.4 lambda by rotating the layers from 90 degrees to 240 degrees, achieving a 42.8% modulation depth, while maintaining radial symmetry, as confirmed by an absolute percentage error analysis (< 9.8%). The demonstrated mechanical tuning mechanism provides a practical pathway toward adaptive THz photonic devices, bridging critical gaps in real-world applications ranging from polarization-encoded communications to depth-resolved biomedical imaging.