A study on polarization performance of aluminum gratings in medium wave infrared

Abstract

Polarization detection in the Medium Wave Infrared (MWIR) region presents broad applications in environmental monitoring, aerial reconnaissance and military target spying. Here, we have fabricated aluminum gratings with 1000 nm and 400 nm pitch by Electron Beam Lithography (EBL) on Si substrate with double sides antireflection coating. The polarization property is characterized by the transmittance and extinction ratio in medium wave infrared measured by the FTIR-based test system. Both Transverse Electric (TE) wave transmittances of Al wire-grid polarizers remain at low value indicating that the TE wave is effectively blocked by the designed grating. The Transverse Magnetic (TM) wave transmittance of Al grating with 400 nm pitch in medium wave infrared augments significantly leading to the increase of extinction ratio and the better polarization performance compared with that of the Al grating with 1000 nm pitch. The average TM transmittance of Al polarization grating with 400 nm pitch exceeds 80% in medium wave infrared which can meet the requirements of polarization detection integrated into medium wave infrared detectors. As the polarization angle of incident light rotates, the transmittances of both Al gratings are of remarkably periodic, which follows Malus’ law. The extinction ratios of Al grating with 400 nm pitch and 1000 nm pitch realize 219:1 and 90:1 at 4.5 μm, respectively. Al wire-grid polarizers manufactured in this paper can be a promising candidate for optical system applications, especially in the field of medium wave infrared detectors.