Enhancing Broadband NUV-Visible Polarization via Aluminum Nanogratings: A Comparative Study of Single and Bilayer Designs

Abstract

Subwavelength-metallic-grating based polarizers have been substantially demonstrated for polarimetric detection in infrared wavelengths. Fewer reports were seen for the applications in near-ultraviolet (NUV) so that achieving both high transmittance and high extinction ratio still remains a challenge. In this work, both single-layer and bilayer aluminum (Al) grating based polarizers operating in the near-ultraviolet (NUV) to visible range are designed and systematically studied. Through finite-difference time-domain (FDTD) simulations, Al is identified as the optimal metal, providing a favorable balance between plasmonic suppression of unwanted polarization and low-loss transmittance. It is further revealed that parabolic cross-section profile of grating lines, commonly formed during fabrication, induce a vertical effective-permittivity gradient that improves impedance matching and enhances transmittance, whereas ideal rectangular profiles provide higher extinction ratios. On the other hand, the bilayer Al/SiO₂/Al grating demonstrates enhanced transmittance, owing to local plasmonic resonance in Fabry-Perot cavity, maintaining strong extinction and improved fabrication tolerance. Related polarizers were fabricated by electron-beam lithography, metallization, lift-off and dry-etching, which exhibit broadband polarization performance consistent with FDTD simulations. These results clarify profile-dependent polarization mechanisms and establish Al nanogratings as a scalable and CMOS-compatible platform for high-performance NUV–visible polarizers.