Prototype demonstration of the broadband anti-reflection coating on sapphire using a sub-wavelength structure

We report the development of the broadband antireflection (AR) coating on sapphire using a sub-wavelength structure for a cosmic microwave background (CMB) polarization experiment. One of the challenges in the CMB polarization experiment is to disentangle the sources of the polarized emission among the CMB and the galactic emissions from our Galaxy. This leads a need of a cryogenically compatible broadband AR coating on a millimeter wave optical element used in a CMB telescope. We design and fabricate the AR subwavelength structure using a laser machining on a sapphire plate, which is a commonly used material as a half-wave plate at millimeter wavelength. We fabricate a prototype sample and evaluate the transmittance of an SWS AR sapphire for the fractional bandwidth of about 1.5. We show the agreement between the measured transmittance and the prediction based on the fabricated shape. The projected transmittance based on the measurement achieves above 90% above 50 GHz. We finally discuss the room of the improvement and the applicability for this technology for a future CMB polarization experiment.