A novel bolometer for infrared and millimeter-wave astrophysics

We are developing a novel bolometer which uses a fine mesh to absorb radiation. The filling factor of the mesh is small, providing a small heat capacity and a low geometric cross-section to cosmic rays. The mesh is patterned from a free-standing silicon nitride membrane and is thermally isolated by long radial legs of silicon nitride. A thin metallic film evaporated on the mesh absorbs radiation by matching the surface impedance to that of free space. A neutron transmutation doped germanium thermistor attached to the center of the mesh detects the temperature increase from absorbed radiation. The low thermal conductivity and heat capacity of silicon nitride provide improved performance in low background applications. We discuss the theoretical limits of the performance of these devices. We have tested a device at 300 mK with an electrical NEP=4×10^-17 W Hz^-1/2 and a time constant τ=40 ms.