Tunable in-plane scanning of focused vector vortex beams via moiré phase engineering in cascaded metasurfaces

Tunable manipulation of focused vector vortex beams (FVVBs) in the terahertz (THz) regime is highly desirable for advanced applications in imaging, sensing, and communications, yet conventional methods often suffer from limited reconfigurability and integration capacity. In this work, we propose a dynamic metasurface platform enabling in-plane scanning of FVVBs through moiré phase engineering in two cascaded, rotatable all-silicon metasurfaces. Each layer is meticulously designed with decomposed phase profiles, combining spiral, gradient, and lens phases, to independently manipulate orthogonal circular polarization (CP) channels. By mechanically rotating the metasurfaces, we numerically demonstrate continuous lateral displacement of the focused vector beam within the focal plane, with negligible variation in focal length and robust performance under inter-layer separation changes. Furthermore, simultaneous rotation of both layers allows arbitrary positioning of the focal spot and tailored polarization symmetry. This strategy offers a versatile and compact solution for dynamic wavefront control, paving the way for adaptive THz systems with high integration capabilities.