Abstract
We report the fabrication and characterization of birefringent terahertz (THz) waveplates produced via fused deposition modeling (FDM) 3D printing using three widely available thermoplastic filaments: polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), and high-impact polystyrene (HIPS). Form-birefringent subwavelength gratings were printed using a 0.2 mm nozzle to tailor the phase retardance. The intrinsic THz optical properties of each polymer were first assessed through terahertz time-domain spectroscopy (THz-TDS) on unstructured slabs, revealing material-dependent absorption. Polarization-resolved THz-TDS was then employed to extract the effective birefringence, retardance, and polarization modulation of the fabricated waveplates. HIPS exhibited the lowest absorption and the most stable birefringent response, followed by ABS, while PLA showed strong absorption and limited polarimetric performance. Measured birefringence spectra agreed with effective medium predictions below the diffraction threshold (0.6 THz); beyond this range, retardance saturation enabled continued phase shifting. Full polarimetric analysis using Stokes parameters confirmed consistent modulation of ellipticity and polarization rotation, particularly in HIPS-based devices. These results provide a comparative framework for selecting printable materials and optimizing designs of low-cost, customizable THz polarization components.