Fabrication and Characterization of a NIR-FIR Dichroic for the Infrared Interferometer BETTII

BETTII is a balloon-borne far infra-red (FIR: 30-100 µm) interferometer that also uses a near-infrared (NIR: 1-2.5 µm) channel for fine pointing sensing using stars. We have developed an inductive grid dichroic to divide the incoming beam into two components, by reflecting FIR light and transmitting NIR light. The dichroic is fabricated using focused electron beam technology to produce a 1 µm period, 100 nm width metal grid on a sapphire substrate in order to have high reflectance for FIR wavelengths. Here we discuss the design and the detailed manufacturing process for such a dichroic. The transmission and reflectance characteristics are also presented. We discuss them in context of the BETTII requirements.