Development and Precision Characterisation Of Broadband Anti-reflection-coated mm-wave Components

Future optical systems in the sub-mm range require low loss, low reflectance and broadband optics. Presented here is the development process for a hot press technique for making broadband multi-layer anti-reflection coatings for plastic lenses and optical components. The elevated temperatures used in this method induce a change in index and mechanical deformation in the substrate material due to polymer chain rearrangements. To avoid this, these components are annealed before they are machined to their intended form and prior to the application of any coating. Study of precise dimensional and refractive index changes between 115-130^◦C across repeated annealing cycles shows that ultra-high molecular weight polyethylene (UHMWPE) requires 3-4 annealing cycles above 125^◦C before stabilisation. We then present 3 and 5 layer test coating recipes for 90-225GHz with comparison between experimental measurements and theory. Preliminary results show good agreement. In this study, the methods presented are kept generic for any sub-mm band focusing broadly on 60-600GHz; however future work will apply what is achieved for future CMB experiments.