Anti-reflection structures for large-aperture cryogenic lenses and vacuum window in 100-GHz band

We are developing a 100-GHz band 109-pixel MKID camera for the Nobeyama 45-m telescope. The camera optics contains plano-convex silicon (Si) lenses with 300- and 154-mm diameters located at the 4-K and 1-K stages, and a vacuum window of 320-mm diameter. Antireflective subwavelength structures (SWSs) for the Si lenses and the vacuum window were designed to reduce surface reflection. Cyclo olefin polymer (COP) was chosen as the base material for vacuum window as the dielectric loss is comparable with high-density polyethylene and it is easy to fabricate. Antireflective SWSs optimized for 100-GHz band were simulated using ANSYS HFSS. A one-layer rectangular pillar was designed for a Si lens of 300-mm diameter and a 320-mm diameter COP window to examine the fabrication process in large areas. For 154-mm diameter Si lens, a 1.2-mm depth tapered structure was used to obtain broadband characteristics. These designed structures were fabricated on both sides using a three-axis numerically-controlled machine. An end mill and a metal-bonded dicing blade were used for cutting the COP and Si, respectively. W-band vector network analyzer was used for S-parameter measurements of the SWS formed flat surface at an ambient temperature. Average surface reflectance of Si lenses and transmittance of the COP window in the 90–110 GHz range were found at approximately 1% and 98%, respectively.