Demonstration of anti-reflective structures over a large area for CMB polarization experiments

Sapphire, alumina, and silicon present the following characteristics that make them suitable as optical elements for millimeter and sub-millimeter applications: low-loss, high thermal conductivity at cryogenic temperatures, and high refractive index ~3. However, the high index also leads to high reflection. We developed a technique to machine sub-wavelength structures (SWS) as a broadband anti-reflection coating on these materials through laser ablation. We describe here the status of our development: transmission measurements of fabricated samples in a diameter of 34.5 mm agree with predictions, and we are now focusing on increasing the fabrication area with high processing rate. This is motivated by the need of ~500 mm diameter optical elements for the next-generation cosmic microwave background polarization experiments. We show our large area machining method on the alumina and sapphire over an area of < 5200 mm2 with the processing rate of < 4:0 mm3=min:, and the transmission measurements are consistent with the predictions.