3D-printed alumina as a millimeter-wave optical element

We present millimeter and sub-millimeter room temperature transmission and loss measurements of 3D-printed alumina disc and of a disc with one-sided 3D-printed sub-wavelength structures anti-reflection coatings (SWS-ARC). For four bands spanning 158 to 700 GHz, we find an index of refraction consistent with n=3.107±0.007. The loss over the entire frequency band between 158 GHz and 700 GHz spans 1·10−3≤tan δ≤2.49·10−3 with 10% to 30% uncertainty at the lower range of frequencies, shrinking to ∼2% at the higher frequencies. As expected, constructive and destructive interference fringes that are apparent with the flat disc data are absent with the disc that has SWS-ARC. The measured data are consistent with finite element analysis predictions that are based on the measured shape of the SWS. At frequencies between 158 and 200 GHz, below the onset of diffraction effects, reflectance is reduced from a maximum of 64% to about 25%, closely matching predictions. These measurements of the index, loss, and SWS-ARC of 3D-printed alumina suggest that the material and fabrication technique could be useful for astrophysical applications.