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>