Cryogenic infrared filter made of alumina for use at millimeter
  wavelength
  We propose a high thermal conductivity infrared (IR) filter using alumina for
use in millimeter wave detection systems. We constructed a prototype two-layer
anti-reflection (AR) coated alumina filter with a diameter of 100mm and a
thickness of 2mm, and characterized its thermal and optical properties. The
transmittance of this filter at 95GHz and 150GHz is 97% and 95% while the
estimated 3dB cutoff frequency is at 450GHz. The high thermal conductivity of
alumina minimizes thermal gradients. We measure a differential temperature of
only 0.21K between the center and the edge of the filter when it is mounted on
a thermal anchor of 77K. We also constructed a thermal model based on the
prototype filter and analyzed the scalability of the filter diameter. We
conclude that temperature increase at the center of alumina IR filter is less
than 6K even with a large diameter of 500mm, when the temperature at the edge
of the filter is 50K. This is suitable for an application to a large-throughput
next-generation cosmic microwave background (CMB) polarization experiment, such
as POLARBEAR-2 (PB-2).
