Design and characterization of a bilayer wire-grid polarizer as a variable-angle polarizing beam splitter

Wire-grid polarizers (WGPs) are subwavelength metallic nanostructures that transmit one linear polarization state and reflect the orthogonal linear polarization state. Bilayer WGPs are devices composed of two sets of wire grids that are vertically offset by a surface-relief dielectric grating. We experimentally demonstrated a silicon-and-aluminum bilayer WGP for operation at wavelengths near 1550 nm that was optimized to be an angularly tolerant polarizing beam splitter (PBS)-to have high efficiency in the transmitted beam and in the orthogonally polarized reflected beam over a wide range of incidence angles. While bilayer WGPs at this wavelength tend to be fabricated with lower-index dielectric materials for better transmission efficiency, using silicon allowed us to both leverage existing silicon processing techniques and to create a device that is compatible with future integration with other silicon-based optical systems. We fabricated two devices, both of which maintained greater than 58% transmittance and greater than 93% reflectance over incidence angles from -50° to +50° for the horizontal polarizer pass axis orientation. Additionally, these devices demonstrated good performance as high contrast transmissive WGPs over this same angular range, maintaining a contrast ratio greater than 1000:1 for the horizontal pass axis orientation. Furthermore, we determined that if the Fresnel reflection at the device substrate's back interface were reduced or eliminated, the transmission efficiency of the bilayer WGP could be as high as 82% for the horizontal pass axis orientation for all measured incidence angles. This demonstrated the potential for a silicon-based bilayer WGP to be used as a high-efficiency, variable-angle PBS that is integration-compatible with other silicon systems.