Broadband plasma sprayed ceramic anti-reflection coating for millimeter-wave astrophysics (Conference Presentation)

The next generations of Cosmic Microwave Background (CMB) polarimetry experiments will attempt to detect the faint primordial B-mode signal from gravitational waves. The increasing scale of photon-noise limited detector arrays of millimeter-wave astrophysics has led to the need for cryogenic refractive optics with large aperture, high dielectric constant, and low loss. Additionally, multiple frequency band observations for galactic foreground removal from CMB signal require broad bandwidth optics. Modern CMB polarimetry experiments use several cryogenically cooled refractive elements made of alumina or silicon. Their high dielectric constants require multiple layers of anti-reflection (AR) coating with different dielectric constants to minimize reflection at the dielectric boundaries. We have developed an AR coating technology for millimeter-wave optics which achieves minimal dissipative loss and broad bandwidth with a simple and accurate fabrication process. Ceramic coatings are applied using a standard plasma spray system. We tune the dielectric constant of the coating by mixing hollow ceramic microspheres with alumina powder as the base material or varying the parameters of the plasma system. By spraying low loss ceramic materials with a tunable dielectric constant, we can apply multiple layers of AR coating for broadband millimeter-wave detection. The ceramic coating also has matching coefficient of thermal contraction with alumina and silicon for robustness to cryogenic delamination. We report on the design, fabrication methodology, and measurement of coating uniformity, repeatability, and transmission at room and cryogenic temperatures. This technology is applicable from submillimeter to millimeter wavelengths for coatings with greater than octave bandwidth.