Monolithic 3D-printed dielectric horn-lens antenna for terahertz waveguide-free space coupling

Horn antennas are pivotal couplers for terahertz (THz) waveguide transmission systems, yet conventional designs usually prioritize the far-field performance mainly considering the long-distance wireless communication and radar applications, often overlooking the near-field beam pattern optimization. Here, we present a monolithic polymer-based THz horn-lens system fabricated via liquid-crystal display stereolithography (LCD-SLA) 3D printing. Through the structurally designed stepped gradient and the geometric focusing advantage of the lens, a near-Gaussian emission beam without sidelobes is achieved. We found that by enhancing the dielectric constant (D_k) of photosensitive resin through Al₂O₃ doping, near-field performance can be further improved. At 30 vol% Al₂O₃, D_k increases from 2.68 to 3.68, leading to a 50% stronger near-field electric field and a 35% smaller focal spot (30 mm working distance). Experimental validation using a THz Doppler velocimetry system confirms the integrated lens antenna retains 68.16% relative signal strength at the distance of 60 mm (220 GHz), outperforming its unmodified counterpart (47.35%). The simulation results of the antenna near-field performances agree well with our measurements. This work advances beam-shaping strategies for waveguide-coupled systems and unlocks high-precision applications in velocity and distance sensing.