Submillimeter wave low pass filters made of glass beads

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Abstract
We have fabricated low pass filters made of spherical glass beads embedded in polyethylene and measured the transmittance at room temperature and 1.3 K as a function of the diameter and the filling factor of the glass beads. The cutoff frequency is a function of the bead diameter. Compared with the transmittance of thin glass plates or Fluorogold filters, these filters have a sharper cutoff due to the effect of scattering by the spherical beads. This effect becomes prominent at low temperature, because of the decrease in the absorption coefficient of glass itself. The glass bead filters have very low transmittance above the cutoff frequency. One of the filters we fabricated has a transmittance of −4 between 50 and 1000 cm−1.