  The desire for higher sensitivity has driven ground-based cosmic microwave
background (CMB) experiments to employ ever larger focal planes, which in turn
require larger reimaging optics. Practical limits to the maximum size of these
optics motivates the development of quasi-optically-coupled (lenslet-coupled),
multi-chroic detectors. These detectors can be sensitive across a broader
bandwidth compared to waveguide-coupled detectors. However, the increase in
bandwidth comes at a cost: the lenses (up to $\sim$700 mm diameter) and
lenslets ($\sim$5 mm diameter, hemispherical lenses on the focal plane) used in
these systems are made from high-refractive-index materials (such as silicon or
amorphous aluminum oxide) that reflect nearly a third of the incident
radiation. In order to maximize the faint CMB signal that reaches the
detectors, the lenses and lenslets must be coated with an anti-reflective (AR)
material. The AR coating must maximize radiation transmission in scientifically
interesting bands and be cryogenically stable. Such a coating was developed for
the third generation camera, SPT-3G, of the South Pole Telescope (SPT)
experiment, but the materials and techniques used in the development are
general to AR coatings for mm-wave optics. The three-layer
polytetrafluoroethylene-based AR coating is broadband, inexpensive, and can be
manufactured with simple tools. The coating is field tested; AR coated focal
plane elements were deployed in the 2016-2017 austral summer and AR coated
reimaging optics were deployed in 2017-2018.
