Two-layer anti-reflection coating with mullite and polyimide foam for
  large-diameter cryogenic infrared filters
  We have developed a novel two-layer anti-reflection (AR) coating method for
large-diameter infrared (IR) filters made of alumina, for the use at cryogenic
temperatures in millimeter wave measurements. Thermally- sprayed mullite and
polyimide foam (Skybond Foam) are used as the AR material. An advantage of the
Skybond Foam is that the index of refraction is chosen between 1.1 and 1.7 by
changing the filling factor. Combination with mullite is suitable for wide-band
millimeter wave measurements with sufficient IR cutoff capability. We present
the material properties, fabrication of a large-diameter IR filter made of
alumina with this AR coating method, and characterizations at cryogenic
temperatures. This technology can be applied to a low-temperature receiver
system with a large-diameter focal plane for next-generation cosmic microwave
background (CMB) polarization measurements, such as POLARBEAR-2 (PB-2).
