Prototype sub-wavelength structure anti-reflection coating on alumina filters for ground-based CMB telescopes

TL;DR

This paper introduces the design and fabrication of sub-wavelength anti-reflection structures on alumina filters, achieving low reflectance and polarization for ground-based CMB telescopes across multiple frequency bands.

Contribution

It presents a novel laser ablation fabrication method for sub-wavelength pyramidal AR structures on alumina, optimized for specific CMB observation bands.

Findings

Achieved in-band reflectance of 2% or less across designed bands.

Fabricated pyramids match predicted shapes and performance.

Instrumental polarization below 0.003.

Abstract

We present designs and fabrication of sub-wavelength anti-reflection (AR) structures on alumina for infrared absorptive filters with passbands near 30, 125, and 250 GHz. These bands are widely used by ground-based instruments measuring the cosmic microwave background radiation. The designs are tuned to provide reflectance of 2% or less for fractional bandwidths between 51% and 72%, with each of the three primary bands containing two sub-bands. We make the sub-wavelength structures (SWS), which resemble a two-dimensional array of pyramids, using laser ablation. We measure the shapes of the fabricated pyramids and show that for incidence angles up to 20 degrees the predicted in-band average reflectance is 2% or less, in agreement with the design. The band average instrumental polarization is less than $3\times 10^{-3}$.