Anisotropic metasurface for off-axis flat-top beam shaping in the terahertz regime

Recently, a plethora of unprecedented studies have been carried out on terahertz flat-top beam shaping with the help of meta-devices. However, the majority of these devices are single-functional and multi-layered, require complicated fabrication processes, and are suitable only for the axial transformation of the input beam. Herein, we propose two bifunctional reflective metasurfaces consisting of anisotropic unit cells etched onto a grounded RO3035 substrate. Aside from transforming a Gaussian beam into a high-quality non-axial flat-top beam at 0.1THz, the first device realizes linear-to-circular polarization (LTCP) conversion, while the second splits the Gaussian beam into two orthogonally polarized tilted flat-top beams. The geometric dimensions of the constitutive building blocks are adjusted based on an amalgamation of the Gerchberg–Saxton algorithm (GSA) and the Rayleigh–Sommerfeld (RS) diffraction formula, which is a novel method, to our knowledge, for phase profile design. The performance of the metasurfaces is evaluated through both the finite element method (FEM) and a memory-efficient LR‑RS method that combines the local response (LR) of the embedded inclusions with the RS diffraction formula. With uniformities of 4.5% to 10.2% and roll-off values between 1dB and 1.1dB, and beam conversion efficiencies exceeding 83.3% across both scenarios, the resulting flat‑top beams are of admissible quality. This positions the proposed metasurface as a promising candidate for compact terahertz imaging and sensing systems, and establishes a new basis for combining polarization conversion and beam splitting with flat‑top beam shaping.