  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.
