Asymmetric twisted bilayer metasurfaces for dynamic directional terahertz wave routing

Multidimensional manipulation of terahertz (THz) waves is essential for advanced communication and sensing, yet existing dynamic devices are fundamentally constrained by reciprocal propagation and coupled control parameters. To overcome these limitations, an all-dielectric cascaded THz reconfigurable metasurface (CTRM) operating at 0.5 THz is proposed to synergistically integrate polarization manipulation, dynamic focusing, and asymmetric light routing. The architecture comprises two all-silicon layers: a geometric-phase-encoded half-wave plate (HWP) and a propagation-phase-encoded quarter-wave plate (QWP). By mechanically rotating the layers, the CTRM achieves continuous switching of polarization states for spatially separated focal spots and enables continuous 360° azimuthal spatial steering that is strictly decoupled from the transmitted polarization. Crucially, by implementing an asymmetric, spin-decoupled phase-encoding scheme in the first layer, the device breaks the spatial symmetry of the cascaded Jones matrix for opposite propagation directions. This enables pronounced direction-dependent asymmetric transmission: forward incidence yields high-fidelity cross-polarized focusing, whereas backward propagation strictly results in randomized, diffuse scattering. This versatile, dynamically tunable platform significantly expands the degrees of freedom in THz optics, offering robust mechanisms for asymmetric information encoding, directional wavefront routing, and adaptive beam steering.