Prototype of meta-surface anti-reflection coating for a 40GHz HDPE lens

For cosmic microwave background (CMB) telescopes, high-density polyethylene (HDPE) is widely used as reimaging lens material on account of its extremely low loss and excellent mechanical properties. The impedance mismatch between free space and lens can cause non-negligible reflection loss and low image quality. An anti-reflection (AR) coating is essential to reduce such effect. Single or multiple layers porous polytetrafluoroethylene (PTFE) membranes are glued to the HDPE lens surface as AR coatings in many previous CMB experiments. However, it is difficult to get a uniform bonding across a curvature surface for a large-aperture CMB telescope. Due to the mismatch of the thermal expansion of the lens and AR coating materials, the AR coatings have delamination or separation problem during cryogenic cycling. Simulated dielectric AR (SDAR) coatings based on the theory of meta-surface are suitable for low frequency and large aperture CMB telescopes. The effective refractive index of SDAR coating is tunable by cutting holes or grooves into the lens material to form sub-wavelength structure. The SDAR coating is made directly from lens bulk material, which can naturally solve the thermal expansion mismatch problem. In this paper, we present the SDAR coating prototype machined on a 30 mm thick HDPE plate. The measurement results show that the SDAR coating can effectively eliminate standing waves with the performance improvement about -10 dB to -20 dB. And the transmittance exceeds 98% in the 30 GHz to 50 GHz band, which is about 10% higher than that without AR coating.