Quasi-Optical Characterization of Low-Loss Polymers at 300 GHz for Vacuum Window Applications

We report on the measurements of the complex refractive index of a number of low-loss polymers using a quasi-optical vector network analyzer operating in the frequency range of 210-370 GHz. The process of extracting both the real and imaginary parts of the refractive indexes is presented in this article, along with the method of error estimation. The measured refractive indexes of 25 different material samples, as well as the estimated error and the loss tangents of thicker samples, are tabulated. For selected materials, loss tangents have also been confirmed using the method of intersecting lines, which measures the additional input noise to a superconductor-insulator-superconductor receiver due to the dielectric sample. The data obtained from this work will be useful for the design of low-loss vacuum windows for cryogenic receivers operating in the 300-GHz range.