Single-sided lens terahertz flat-top beam shaping in the Fresnel region

Inthis paper, a novel analytical approach is introduced for the design of a single-sided plano-convex lens (PCL) capable of terahertz (THz) flat-top beam shaping in the Fresnel region. The lens profile, derived from geometrical optics (GO) principles, integrates collimating and beam-shaping functions into a single optical surface, thereby simplifying fabrication. The efficiency of the method is demonstrated by designing two PCLs that convert a divergent Gaussian beam from a monochromatic THz source (at 100 GHz and 200 GHz) into flat-top beams with target radii of 400 mm and 500 mm at distances of 3 m and 4 m, respectively. The resulting f ields, evaluated through analytical, integral equation (IE), and finite element (FEM) techniques, are shown to be comparable to those produced by a conventional double-sided concave-convex lens (CCL). Furthermore, the sensitivity of the design to frequency and alignment variations is investigated, confirming the robustness of this single-curvature approach for integration into practical THz imaging systems.