Reconfigurable transmissive metasurface with a combination of scissor and rotation actuators for independently controlling beam scanning and polarization conversion

The study addresses the challenge of integrating polarization conversion and beam scanning within a single transmissive metasurface platform while enabling independent control of these two functions. Reconfigurable metasurfaces manipulate electromagnetic wave properties (amplitude, phase, polarization) and, in transmissive mode, direct transmitted waves. Polarization conversion mitigates polarization mismatch by converting linear to circular states (RHCP/LHCP), and beam scanning steers waves for improved imaging, radar, and communications. Prior approaches often suffer from narrow scanning ranges, frequency dependence, limited ability to switch polarization states, lack of independent control, and fabrication complexity, especially for transmissive structures. The paper proposes a mechanically reconfigurable transmissive metasurface that decouples polarization conversion and beam steering via rotation and scissor actuators, respectively, aiming for practical, low-cost, and adaptable operation at 10.5 GHz.

Conventional reconfigurable metasurfaces achieve beam steering and polarization conversion through electrical, thermal, or mechanical phase and spacing modulation. Phase modulation changes element conductivity or geometry to impart phase shifts; spacing modulation manipulates inter-element distance to steer beams. Numerous designs combining these functions have shown sophisticated control, including human-mind-based manipulation, but remain constrained by narrow scan ranges, polarization switching limitations, frequency sensitivity, and lack of independent control of functions. Designing efficient transmissive metasurfaces adds further complexity. Applications motivating these functions include imaging, radar, and communications. The paper situates its contribution within this landscape by employing spacing modulation for beam scanning and Pancharatnam-Berry phase via UC rotation for polarization conversion, with independent mechanical actuation.

Design concept: A transmissive metasurface combines a rotation actuator and a scissor actuator to independently control polarization conversion and beam scanning. The array consists of four types of two-bit rotatable unit cells (UCs) labeled A, B, C, and D. Flipping/rotating UCs reverses the phase distribution to switch between RHCP and LHCP, while a scissor mechanism linearly varies the inter-UC spacing d for beam steering. Analytical model: Beam steering via spacing modulation is analyzed using the array factor AF(θ) = Σ_{n=1}^N exp(j(n−1)d(θ) + jφ_n), where d is the inter-UC spacing and φ_n is the UC phase. Nonplanar UC geometry enlarges the achievable variation range of d by allowing overlap and rotation, effectively forming two virtual layers whose AFs sum to predict the beam angle. With λ0 = 28.5 mm (10.5 GHz), and considering α ∈ [0°, 23.2°], the practical d range is [0.25λ0, 0.65λ0], yielding an analytically estimated beam steering angle span from about 18° to 60°. Unit cell design and electromagnetic properties: Each UC is a three-layer I-shaped frequency selective surface on FR4, configured for transmissive operation near 10.5 GHz. Key dimensions: r = 3.8 mm, a = 6.4 mm, b = 1.4 mm. Simulations indicate average transmission better than −2 dB at 10.5 GHz with bandwidth 10.15–10.65 GHz. Transmission sensitivity: magnitude decreases from −1.1 dB to −1.8 dB as FR4 tanδ increases 0.005→0.02; for tanδ = 0 (lossless) it is −0.9 dB. Oblique incidence degrades transmission from −1.8 dB (0°) to −4.1 dB (25°). At α = 30°, average transmission loss and phase excursion are approximately −3.3 dB and 20°. Full-wave simulation: ANSYS HFSS is used with master–slave periodic boundaries and a Floquet port excitation (PML boundaries in far-field simulations). To avoid conductor shorting in overlapping patterns, the UC rotation angle is modeled as α = α0 + k with a small offset k = 1°. Two polarization modes are defined by UC sequence: Mode M1: A–B–C–D repeating; Mode M2: flipped sequence A–D–C–B repeating. With spacing modulation d ∈ [0.25λ, 0.65λ], metasurface far-field patterns are computed at 10.5 GHz under plane-wave excitation. Experimental prototype and measurements: A 12×12 UC array was fabricated on FR4. A scissor actuator adjusts UC spacing to d = 0.35λ, 0.50λ, and 0.60λ. A rotation actuator sets the UC sequence for modes M1 and M2 to switch RHCP/LHCP orientation. Measurements employ two horn antennas: a transmitting horn behind the metasurface producing a linearly polarized plane wave; and a receiving horn in front measuring horizontal and vertical polarization components to reconstruct RHCP/LHCP radiation patterns. An Anritsu MS2038C VNA records magnitude and phase. Patterns are normalized with 5° angular resolution. Measurement setup details are provided in supplementary materials.

- Independent control: The rotation actuator switches the polarization state (RHCP↔LHCP) by flipping UC sequence between modes M1 and M2, while the scissor actuator independently steers the beam via spacing modulation d. - Analytical prediction: With nonplanar UCs and d ∈ [0.25λ, 0.65λ], predicted steering angles span roughly 18°–60° using a two-layer AF model. - Simulation (10.5 GHz): For mode M1 at fixed d = 0.50λ, RHCP and LHCP beams are generated on opposite sides of broadside. As d varies from 0.25λ to 0.65λ and α < 30°, RHCP and LHCP scan from −50° to −22° and +50° to +22°, respectively (scan window ~28°). For mode M2 (flipped UC order), the RHCP/LHCP sides swap, and scanning ranges mirror those of M1. The simulated scan span (28°) is 14° narrower than the analytical estimate, attributed to phase changes under UC rotation. - Transmission and bandwidth: Average transmission around 10.5 GHz is better than −2 dB; 3-layer UC bandwidth is approximately 10.15–10.65 GHz. Loss increases with FR4 tanδ and with incidence angle (−1.8 dB at normal to −4.1 dB at 25°). - Measurements (12×12 prototype): For both M1 and M2, measured RHCP/LHCP beam angles closely match simulations at d = 0.35λ and 0.60λ (≈37° and 22°, respectively). At d = 0.50λ, RHCP in M1 shows ~1° deviation from the simulated 27°, attributable to fabrication tolerances and setup. - Overall: Numerical and experimental results confirm independent polarization conversion and beam scanning with a scan range of about 28° on each side at 10.5 GHz.

The work demonstrates a transmissive metasurface architecture that decouples beam steering from polarization conversion via independent mechanical actuators. Spacing modulation using a scissor mechanism enables continuous beam scanning, while UC rotation realizes Pancharatnam-Berry phase control to switch between RHCP and LHCP without altering beam steering. Analytical models predicted wide-angle steering when leveraging nonplanar UC geometry; full-wave simulations and measurements validated robust scanning behavior but with a reduced span relative to the idealized prediction due to rotation-induced phase variations and material/angle losses. The measured agreement across multiple spacings and both polarization modes confirms the practical feasibility of independent control, addressing longstanding constraints (narrow scans, coupled functions) in reconfigurable metasurfaces. The approach is well-suited for scenarios where tuning speed is not critical (e.g., biosensing, wireless power transfer, indoor communications) and may be adaptable to larger arrays and different frequencies.

The paper introduces a mechanically reconfigurable transmissive metasurface that combines a scissor actuator for spacing modulation with a rotation actuator for Pancharatnam-Berry phase control, enabling independent beam scanning and polarization conversion. Simulations and a 12×12 prototype at 10.5 GHz confirm RHCP/LHCP switching and approximately 28° scanning per side over d ∈ [0.25λ, 0.65λ], with measured beam angles closely matching simulations at multiple spacings. This architecture offers a practical, low-cost route to multifunctional transmissive metasurfaces and is promising for applications not requiring rapid tuning. Future work could pursue: lower-loss substrates to improve transmission efficiency; angle-insensitive UC designs via optimization; expanded bandwidth through additional layers; faster actuation mechanisms; and scaling to larger apertures and higher frequencies for satellite and advanced communication systems.

- Mechanical actuation leads to slower tuning compared to electronically reconfigurable metasurfaces. - Measured/simulated scan span (~28°) is narrower than the analytical prediction, likely due to rotation-induced phase variations and practical constraints. - Use of FR4 (tanδ ≈ 0.02) limits transmission efficiency; significant degradation occurs at oblique incidences. - Demonstrated bandwidth is modest (≈10.15–10.65 GHz); broader bandwidth would require additional layers or alternative designs. - Angle sensitivity and potential conductor shorting in overlapping regions necessitate design offsets (k = 1°) and may constrain extreme configurations. - Results are shown for a 12×12 array at a single operational frequency; generalizability to different sizes/frequencies requires further validation.