Author list: Ryota Takaku, Shaul Hanany, Hiroaki Imada, Hirokazu Ishino, Nobuhiko Katayama, Kunimoto Komatsu, Kuniaki Konishi, Makoto Kuwata-Gonokami, Tomotake Matsumura, Kazuhisa Mitsuda, Haruyuki Sakurai, Yuki Sakurai, Qi Wen, Noriko Y. Yamasaki, Karl Young, and Junji Yumoto
Journal: Journal of Applied Physics 128, 225302 (2020) (JAP, arXiv)
Abstract: We designed, fabricated, and measured anti-reflection coating (ARC) on sapphire that has 116% fractional bandwidth and transmission of at least 97% in the millimeter wave band. The ARC was based on patterning pyramid-like sub-wavelength structures (SWS) using ablation with a 15 W femto-second laser operating at 1030 nm. One side of each of two disks was fabricated with SWS that had a pitch of 0.54 mm and a height of 2 mm. The average ablation volume removal rate was 1.6 mm3/min. Measurements of the two-disk sandwich show transmission higher than 97% between 43 and 161 GHz. We characterize instrumental polarization (IP) arising from differential transmission due to asymmetric SWS. We find that with proper alignment of the two-disk sandwich, RMS IP across the band is predicted to be 0.07% at normal incidence and less than 0.6% at incidence angles up to 20°. These results indicate that laser ablation of SWS on sapphire and on other hard materials such as alumina is an effective way to fabricate broadband ARC.
