Fabrication and optical characterization of a silicon bilayer wire-grid polarizer at a wavelength of 1550nm

Abstract

Wire-grid polarizers (WGPs) are subwavelength metallic nanostructures that transmit the transverse magnetic (TM) linear polarization state and reflect the transverse electric (TE) linear polarization state. BilayerWGPs are devices composed of two sets of wire grids that are vertically offset via a surface-relief dielectric grating. Bilayer WGPs generally have higher extinction ratios for transmitted light, higher efficiency of the reflected state, and lower angular sensitivity than standard WGPs. We experimentally demonstrate an aluminum and silicon bilayer WGP optimized for operation at a wavelength of 1550nm. While bilayer WGPs at this wavelength tend to be fabricated with lower-index dielectric materials for better transmission efficiency, using silicon allows us to leverage existing expertise in nanoscale silicon etching methods and to create a device that is compatible with future integration into microoptoelectromechanical systems (MOEMS). This demonstration device achieved a measured TM transmission efficiency of 58.8% at normal incidence with a transmission extinction ratio of at least 2400:1. If the loss due to Fresnel reflection at the silicon substrate’s back interface is accounted for, we calculate that the bilayer grating’s TM transmission efficiency is greater than 80% at normal incidence. The measured TE reflection efficiency was greater than 94.5%, which is comparable to reported reflection efficiencies of bilayer WGPs on lower-index substrates since the TE reflectance is primarily driven by interaction with the metallic nanostructures. Therefore, with the addition of an antireflection coating on the back of the silicon substrate, these bilayer devices could achieve high TM transmission efficiency, high TE reflection efficiency, and a high transmissive extinction ratio, while being compatible for integration into MOEMS devices.