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 100 mm and a thickness of 2 mm , and characterized its thermal and optical properties .
The transmittance of this filter at 95 GHz and 150 GHz is 97 % and 95 % while the estimated 3 dB cutoff frequency is at 450 GHz .
The high thermal conductivity of alumina minimizes thermal gradients .
We measure a differential temperature of only 0.21 K between the center and the edge of the filter when it is mounted on a thermal anchor of 77 K. 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 6 K even with a large diameter of 500 mm , when the temperature at the edge of the filter is 50 K. This is suitable for an application to a large-throughput next-generation cosmic microwave background ( CMB ) polarization experiment , such as POLARBEAR-2 ( PB-2 ) .