Optimization study of metallic sub-wavelength gratings as the polarizer in infrared wavelengths

Despite the polarimetric detection in the infrared wavelengths of 8-10 µm being of great importance and broad applications, there has been limited addressing of the grating-based polarizers in this band. One of the main issues lies in the process incompatibility between the conventional nanofabrication technique and the II-VI materials such as HgCdTe, so that the direct integration of polarizers with sensors still remains a big challenge. This paper reports our recent work on optimizing the grating structures, materials, and nanofabrication processes for enhancing both the transmittance and the extinction ratio of polarizers on Si and/or ZnSe wafers, using numerical simulations for the grating design and electron beam lithography for the nanoscale pattern generation. By utilizing the finite-difference time-domain method, both the transmittance and the extinction ratio are maximized by optimizing the grating geometric dimensions and the duty cycle for two different grating materials of Al and Au for comparison. Based on the designed structures, nanofabrications of sub-wavelength gratings in both Al and Au are carried out, and the processes are compared for achieving high polarization performance. Optical characterizations of the fabricated polarizers demonstrate that both high transmittance and extinction ratio can be achieved in feasible parameters and the nano-process developed in this work.