Anti-reflection Coating for Cryogenic Silicon and Alumina Lenses in Millimeter-Wave Bands

• DOI: 10.1007/s10909-013-1059-3
• Publication Date: 2014-01-13T10:24:48Z
• Authors: Tom Nitta; Shigeyuki Sekiguchi; Yutaro Sekimoto; Kenji Mitsui; Norio Okada; Kenichi Karatsu; Masato Naruse; Masakazu Sekine; Hiroshi Matsuo; Takashi Noguchi; Masumichi Seta; Naomasa Nakai
• Keywords: Anti-reflective coating; Extremely high frequency; Optical coating

A dielectric lens with high refractive index is suitable for focusing cryogenic devices in millimeter-wave bands when an appropriate anti-reflection (AR) coating is applied. Two types of AR coatings for silicon and alumina were studied at the millimeter-wave (220 GHz) band: one is by direct machining of mixed epoxy for a silicon lens array, while the other is by laser machining of an antireflective subwavelength structure for a large alumina lens used in a re-imaging optics system. The millimeter-wave optical properties of silicon, alumina, aluminum nitride, and Stycast epoxies were measured with a Fourier Transform Spectrometer (FTS) at cryogenic temperatures. The measured refractive index of the AR coating with a mixture of Stycast 1266 (n = 1.68) and Stycast 2850FTJ (n = 2.2) for silicon at 30 K was 1.84. The thickness of the epoxy AR coating was precisely controlled with direct machining. Transmittance of the AR-coated silicon substrate, measured with FTS, was approximately 95 % at the center frequency of the 220 GHz band with a bandwidth of 25 % at 27 K. An antireflective subwavelength structure was designed for an alumina sample with periodic cylindrical holes. The measured 220-GHz-band transmittance was above 90 % with a bandwidth of 25 % at 25 K.