Extreme‐Thickness Meta‐Membrane for Controlling Terahertz Vectorial Beams

Manipulating terahertz (THz) beams at will in transmission mode is desired, but conventional devices are bulky and unfavourable for optical integration. Metasurfaces (MSs) offer a versatile platform for light manipulation, but transmission‐mode MSs need sufficient thickness to achieve wide‐range phase modulation. This work proposes a new type of flexible MSs, consisting of two metallic screens with air slits exhibiting tailored geometries and orientations, to manipulate the wave‐fronts and polarization properties of THz beams in transmission mode. In contrast to conventional Huygens MSs involving impedance‐matched electric and magnetic resonances, our meta‐system relies on near‐field coupling of two slit resonances and thus can be extremely thin (∼λ/30). This work employs leaky eigenmode theory to analyse the transmission properties of arrays of coupled air slits and experimentally demonstrates that the phase and polarization of the transmitted THz wave can be modulated by varying the structures and orientation angles of two air slits. Using these coupled air‐slits as meta‐atoms, three THz MSs are constructed to generate scalar and radially polarized vectorial Bessel beams under 0.39 THz illumination. The results are confirmed by full‐wave simulations. This approach enables transmissive THz metadevices for applications such as biosensing, communications, and displays.