Terahertz Optical Properties of 3D-Printed Samples with Variable Infill Factors for Flat Gradient-Index Lens Design

<p class="MsoNormal"><span lang="EN-GB">The study presents a comprehensive characterization of 3D-printed gradient-index (GRIN) lenses with subwavelength fill-factor modulation operating in the terahertz (THz) radiation range. The analysis focused on four infill geometries (aligned rectilinear, rectilinear, gyroid, honeycomb) fabricated using cyclic olefin copolymer (COC) via fused deposition modelling (FDM). The optical properties of the infill structures were measured using THz time-domain spectroscopy (THz-TDS) with the TeraPulse Lx Modular System (TeraView) operating in transmission mode within a dry-air chamber. The refractive index and absorption coefficient of samples were determined for fill factors ranging from 20% to 90% in 10% increments within the frequency range of 100 GHz to 2 THz. Based on measured optical properties, GRIN lenses operating at 96.4 GHz were designed and fabricated with three discrete phase-quantization configurations (4-, 8-, 16 binarization-level) for each infill geometry. The experimental results demonstrate that the aligned rectilinear 8-level GRIN lens achieved optimal performance. All tested configurations exceeded 70% of their theoretical efficiencies, validating the multi-level phase binarization approach as a practical alternative for THz optical applications.</span></p>