arXiv:2611.06050v1 Announce Type: cross Abstract: In this paper, a novel, to our knowledge, 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 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 and 200 GHz) into flat-top beams with target radii of 400 and 500 mm at distances of 3 and 4 m, respectively. The resulting fields, evaluated through analytical, integral equation, and finite element techniques, are shown to be comparable to those produced by a conventional double-sided concave-convex lens. 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.