Optimizacija oblike površinskih struktur širokopasovnih mikrovalovnih absorberjev za uporabo v eksperimentih za opazovanje mikrovalovnega sevanja ozadja

Observations of the cosmic microwave background polarization are an important field in cosmology. These observations, conducted both from Earth and space, deal with extremely weak signals that require very precise detectors. Microwave absorbers are used to cover the supporting structures of the telescope to minimize noise caused by unwanted reflections. These absorbers consist of plastic tiles with additives (carbon), featuring pyramids with square unit cells on their absorbing side. The pyramids reduce the reflections at the interface between the absorber and vacuum caused by a mismatch in refractive indices between the vacuum and the absorber. When the pyramids are small compared to the microwave wavelength, they can be modeled using effective medium theory as impedance tapers. If they are larger than the wavelength, the waves reflect multiple times between individual pyramids. An impedance taper is a function that gradually changes the impedance from the impedance of free space to that of the absorber, reducing the reflected radiation (reflectance). Five different impedance tapers were tested. For absorber fabrication, two 3D printing methods, stereolithography and fused filament fabrication, were evaluated, along with the use of composite resin. Fused filament fabrication using ESD HIPS filament was selected. The reflectance of the manufactured absorbers was modeled using the transfer matrix method and simulated using the finite element method. Reflectance was measured with the Toptica TeraScan 1550 THz spectrometer at 100--400~GHz. Because it was not possible to fabricate absorbers with a sufficiently small unit cell to satisfy the assumptions of the effective medium theory, the theoretically estimated reflectances deviated from the measurements. In the regime with a unit cell larger than the effective medium theory limit, non-specular scattering (scattering in angles other than incidence) in several discrete directions occurs. The fabricated absorbers achieved reflectance comparable to that of other state-of-the-art absorber designs. Significant differences in reflectance were observed between absorbers corresponding to different impedance tapers, highlighting the need for further optimization of the shape of surface structures.