Broadband antireflection coatings adopting 3-layer homogeneous dielectric materials for birefrigent sapphire waveplate in millimeter-wave band

The Design, fabrication, and measurement of a broadband 3-layer antireflection (AR) coating on birefrigent sapphire waveplate in the cryogenic environment are reported in this paper. The proposed ARC design laminates 3 different uniform dielectric materials (n=1.25, 1.81, and 2.45) and bonds them together with the sapphire waveplate in the vacuum heating oven in the cleanroom. The transmission of coated waveplate in both fast and slow axis are greatly improved from averagely around 55.8% to 94.8% and from 61.2% to 98.8% in two axis across from 105 GHz to 300 GHz (the relative bandwidth approaching 100%), meanwhile the ARC also has the merit of incident angle robustness for the homogeneous property of the dielectrics. The experimental results are in excellent agreement with theoretical calculations after the curve fitting technique. The proposed design method of ARC reported in this paper provides us a novel and straightforward method to design the recipe of ARC using conventional dielectric materials, which could be potentially applied for various waveplates and other devices made of any type of material.