Reflective terahertz metalens with hybrid phase design for high-efficiency off-axis focusing

A reflective terahertz metalens, optimized at 0.89 THz, capable of high-efficiency off-axis Bessel beam focusing is proposed and numerically demonstrated. The design utilizes a hybrid phase profile combining conical and hyperbolic components, computed using a vector-based formulation for precise wavefront control. Simulations confirm stable transverse confinement and an extended depth of focus exceeding 20 λ for deflection angles of 5°, 10°, and 15°. Our metalens maintains focusing efficiencies above 70% in all cases, validating the robustness of the design. The proposed approach offers a compact and effective solution for off-axis focusing in terahertz systems, with potential for integration in advanced imaging, beam steering, and spectroscopic devices.