3D Printed Alumina as a Millimeter-Wave Optical Element

TL;DR

This study demonstrates the use of 3D printed alumina as a millimeter-wave optical element, showing its suitable optical properties and potential for astrophysical applications through detailed transmission, loss, and anti-reflection measurements.

Contribution

First comprehensive characterization of 3D printed alumina's millimeter-wave optical properties, including index, loss, and anti-reflection coatings, for potential astrophysical use.

Findings

Index of refraction n ≈ 3.107 across 158-700 GHz

Loss tangent between 1×10⁻³ and 2.49×10⁻³ with frequency-dependent uncertainty

Anti-reflection structures effectively reduce reflectance from 64% to 25%

Abstract

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 - 700~GHz we find an index of refraction consistent with $n= 3.107 \pm 0.007$. The loss over the entire frequency band between 158~GHz and 700~GHz spans $ 1 \cdot 10^{-3} \leq \tan \delta \leq 2.49 \cdot 10^{-3}$ with 10%-30% uncertainty at the lower range of frequencies shrinking to $\sim\!2\%$ 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~GHz 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.