KEYWORDS: Reflectivity, Transmittance, Mirrors, Monte Carlo methods, Fourier transforms, Refractive index, Error analysis, Terahertz radiation, Signal to noise ratio, Interferometers
In many astrophysical instruments, optical elements and absorbers are extensively used and can alter the status of input polarized light. In particular in mm-wave experiments the lack of knowledge of material properties is becoming the most critical issue in controlling systematic effects. We developed a pipeline of test and analysis to characterize absorbing materials through the measurement of reflectance and transmittance spectra over a broad range of frequencies (from 200 GHz to 800 GHz) and incident angles (up to 70°). Spectra were acquired with a Martin-Puplett interferometer, coupled with a cryogenic bolometer detector. The setup is characterized by using a Neoprene sample (1 mm thick). The aliasing, the reproducibility and the incidence angle error are analyzed to evaluate the impact on the measurements. Individual one-day sessions ensure the capability to characterize transmittance/reflectance with high signal-to-noise: error < 0.5% in the 300-800 GHz band and ∼ 2% in the 200-300 GHz. A Monte Carlo-based fitting method is used to retrieve the physical properties of the sample. The thickness 1.005±0.016 mm is compatible with the one measured with calipers. The measured refractive index at 200 GHz is n=2.416±0.032.
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