Development of a broadband anti-reflection coating for use in a cosmic microwave background polarization experiment

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The measurements of the cosmic microwave background polarization can provide the tool to probe the inflationary paradigm. One of the key features in the measurements is to cover the broad bandwidth and the corresponding optical system. Traditionally the broadband anti-reflection coating is done by applying the multi-layer on an optical components. At the cryogenic temperature the multi-layer coating tends to peel off and this has never be established as a robust technology. We propose to fabricate a moth-eye structure on an optical element. We investigate the machinability on a material such as rexolite and alumina by using the dicing saw and laser machining. We also constructed the measurement setup to evaluate the transmission and polarization properties of a prototype sample. The moth-eye structures on a rexolite show the agreement between the predicted performance and the measurements. We also evaluate the prospect of a moth-eye machining for a larger sample, a diameter of 300~mm.