title: Large-aperture wide-bandwidth antireflection-coated silicon lenses for millimeter wavelengths.
abstract: The increasing scale of cryogenic detector arrays for submillimeter and millimeter wavelength astrophysics has led to the need for large aperture, high index of refraction, low loss, cryogenic refracting optics. Silicon with n=3.4, low loss, and high thermal conductivity is a nearly optimal material for these purposes but requires an antireflection (AR) coating with broad bandwidth, low loss, low reflectance, and a matched coefficient of thermal expansion. We present an AR coating for curved silicon optics comprised of subwavelength features cut into the lens surface with a custom three-axis silicon dicing saw. These features constitute a metamaterial that behaves as a simple dielectric coating. We have fabricated silicon lenses as large as 33.4 cm in diameter with micromachined layers optimized for use between 125 and 165 GHz. Our design reduces average reflections to a few tenths of a percent for angles of incidence up to 30° with low cross polarization. We describe the design, tolerance, manufacture, and measurements of these coatings and present measurements of the optical properties of silicon at millimeter wavelengths at cryogenic and room temperatures. This coating and lens fabrication approach is applicable from centimeter to submillimeter wavelengths and can be used to fabricate coatings with greater than octave bandwidth.
authors: R
authors: R
authors: Datta
authors: C D
authors: CD
authors: Munson
authors: M D
authors: MD
authors: Niemack
authors: J J
authors: JJ
authors: McMahon
authors: J
authors: J
authors: Britton
authors: E J
authors: EJ
authors: Wollack
authors: J
authors: J
authors: Beall
authors: M J
authors: MJ
authors: Devlin
authors: J
authors: J
authors: Fowler
authors: P
authors: P
authors: Gallardo
authors: J
authors: J
authors: Hubmayr
authors: K
authors: K
authors: Irwin
authors: L
authors: L
authors: Newburgh
authors: J P
authors: JP
authors: Nibarger
authors: L
authors: L
authors: Page
authors: M A
authors: MA
authors: Quijada
authors: B L
authors: BL
authors: Schmitt
authors: S T
authors: ST
authors: Staggs
authors: R
authors: R
authors: Thornton
authors: L
authors: L
authors: Zhang