Multi-layer anti-reflection coats using ePTFE membrane for mm-wavelength plastic optics

TL;DR

This paper introduces multilayer anti-reflection coatings using layered ePTFE membranes for millimeter-wave plastic optics, enabling broad bandwidths and low reflectance for high-frequency applications like the BICEP Array receiver.

Contribution

It presents a novel multilayer AR coating technique with layered ePTFE membranes that achieves low reflectance and broad bandwidths for millimeter-wave plastic optics.

Findings

Achieved band average reflections between 0.2% and 0.6%.

Demonstrated coating process on large-area optics up to 33cm radius.

Enabled wide parameter space for AR coatings with variable densities and thicknesses.

Abstract

Future millimeter wavelength experiments aim to both increase aperture diameters and broaden bandwidths to increase the sensitivity of the receivers. These changes produce a challenging anti-reflection (AR) design problem for refracting and transmissive optics. The higher frequency plastic optics require consistently thin polymer coats across a wide area, while wider bandwidths require multilayer designs. We present multilayer AR coats for plastic optics of the high frequency BICEP Array receiver (200-300 GHz) utilizing an expanded polytetrafluoroethylene (ePTFE) membrane, layered and compressively heat-bonded to itself. This process allows for a range of densities (from 0.3g/cc to 1g/cc) and thicknesses (>0.05mm) over a wide radius (33cm), opening the parameter space of potential AR coats in interesting directions. The layered ePTFE membrane has been combined with other polymer layers to produce band average reflections between 0.2% and 0.6% on high density polyethylene and a thin high modulus polyethylene window, respectively.