Broadband spectral characterization of lossy dielectrics for mm/submm optical applications

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.