In order for additively manufactured terahertz quasi-optics to meet a necessary condition of combined optics and manufacturing, the operating wavelength determines the permissible scales of features and surfaces, while the material path length defines whether the absorption prevents the transmission. A solution in printing smooth enough geometry will fail due to the loss of resin in case of millimetre-thick lens, and a solution in transparent polymer printing will fail if the scale of features printed is too large compared to the operating wavelength. The calculations presented above establish the required trade-off for material extrusion, material jetting, vat photopolymerization and powder bed fusion using nineteen printable materials, five composite property paths, ten terahertz frequencies and three optical path lengths. Wavelength-normalized feature ratios, wavelength-normalized roughness ratios and Beer-Lambert transmission through 0.5, 1.0 and 2.0 mm paths have been found according to the data presented in the tables. The best absorption-limited transmission belongs to material extrusion in terms of low-loss filaments, which is 0.905 through 1 mm path for $\alpha =1$ cm$^{-1}$, while its mean feature size 415.17$\mu$m sets its wavelength-based geometry limit at 0.72 THz. Vat photopolymerization ensures the largest possible geometry range, which is 3.72 THz in terms of wavelength-based feature size and 10.56 THz in terms of wavelength-based roughness, however, 1 mm transmission is only 0.150 for the smallest process-level resin absorption coefficient of 19 cm$^{-1}$. Material jetting provides the largest build-rate value while loosing majority of transmitted intensity in case of millimetre-scale paths. Powder bed fusion is reserved for reflective or structural terahertz components until transmissive dielectric constant values of printed material are determined. In summary, the proposed selection criterion is tailored for the particular component: thick optics around 1 THz must rely on low loss extruded COC, HDPE or PS, while surface-relief and thin phase structures at 2-3 THz require sub-micron resolution with careful management of resin loss and optical path length.

terahertz optics,additive manufacturing,optical thickness,material extrusion,vat photopolymerization,surface roughness,absorption coefficient,quasi-optics