Large diameter millimeter-wave low-pass filter made of alumina with laser ablated anti-reflection coating

We fabricated a 302 mm diameter low-pass filter made of alumina that has an anti-reflection coating (ARC) with laser-ablated sub-wavelength structures (SWS). The been integrated into and is operating the MUSTANG2 instrument, which coupled to Green Bank Telescope. average transmittance in band between 75 105 GHz 98%. Reflective loss due ARC 1%. difference transmission s- p-polarization states less than To within 1% accuracy we observe no variance these results when measured six independent spatial locations. replaced prior Teflon filter. Data taken heat sunk its nominal 40 K stage show performance consistent expectations: reduction about 50% filters-induced optical power load on 300 mK stage, in-band loading detectors. It 4 days laser-ablate SWS both sides disk. This first report deployed demonstration large area fabrication laser ablation.