Process and material constraints of additive manufacturing for fabrication of terahertz quasi-optical components

• Many commonly used 3D-printable materials exhibit absorption at THz frequencies. • Photopolymers strongly absorb THz radiation, limiting high-resolution processes. • Composites exhibit unique THz spectral properties. • Quasi-optical components require fine features and smooth surfaces. • Material extrusion lacks the required resolution to produce quasi-optics. Additive Manufacturing (AM) is an emerging fabrication method for terahertz-frequency (THz) quasi-optical components, allowing for the creation of complex geometries at an unprecedented short lead time whilst being a cost-effective production method. However, many 3D-printable materials have sub-optimal material properties at THz frequencies, while several AM processes lack the resolution required to produce optical components suitable for operation above 1 THz. This work reviews the THz response of a range of AM materials, analysing their optical properties. Additionally, relevant AM process factors for manufacturing THz optics are evaluated, including the achievable minimum feature size, surface roughness and build rates, across AM process categories. Studies demonstrate that AM processes capable of fabricating sub-100 µm features, including vat photopolymerization, have the spatial resolution necessary for optics operating at frequencies above 1 THz. However, they have a limited material choice of absorptive photopolymer resins. In contrast, material extrusion enables access to a wider range of materials, including non-polar polymers that are highly transparent at THz frequencies. However, it offers poor resolution and produces rough surfaces relative to the wavelength of the THz region. This review has identified several areas for future investigation, including the THz characterisation of AM-manufactured ceramics and composites, which could offer desirable properties for THz optics. This review comprehensively examines the requirements and constraints of Additive Manufacturing (AM) technologies as a solution to produce optical components for terahertz systems. It assesses the properties of AM feedstock materials, examining their refractive index and absorption coefficient at THz frequencies. AM process categories are also evaluated based on the achievable resolution, processing rate and surface roughness.