Laser ablated sub-wavelength structure anti-reflection coated alumina filter for Taurus high frequency telescope

We report on the design, fabrication, and measurements of a prototype alumina filter for the high frequency dichroic band 235 - 405 GHz of the Taurus balloon-borne telescope. Taurus is a cosmic microwave background polarimetric imager aiming to produce maps of the CMB E-mode polarization. The 5 cm diameter and 2 mm thick alumina filter prototype has a sub-wavelength structure (SWS) anti-reflection coating with measured average reflectance for the 235 – 405 GHz dichroic band of 1.2%. The SWS were fabricated using femtosecond laser ablation. To our knowledge, this is the highest frequency SWS anti-reflection coating on an alumina substrate.