Integrating subwavelength polarizers onto InP-InxGa1−xAs sensors for polarimetric detection at short infrared wavelength

Abstract

There are growing needs for polarimetric detection at infrared wavelengths for broad applications in bioscience, communications, ocean detection, agriculture, etc. Subwavelength metallic gratings are capable of separating the transverse magnetic mode from the transverse electric mode to form polarized light, offering a promising approach for realizing polarimetric detection. The conventional method is to assemble a commercially supplied polarizer in the form of a chip in a standard detector. This article describes our recent advances in integrating a subwavelength grating with an independent InP-InxGa1−xAs sensor by nanofabrication based on electron-beam lithography. A technical issue encountered in the fabrication is the strong backscattering of primary electrons from InP-InxGa1−xAs, causing significant extra exposure on the resist such that the standard copolymer/poly(methyl methacrylate) bilayer is not applicable. Instead, a single layer of poly(methyl methacrylate) is applied, and the necessary undercut profile for the lift-off process is created by use of the beam spread condition in the lithography. The fabricated detector with pixelated polarizers demonstrates uniform polarimetric detection at a wavelength of 1320 nm with an extinction ratio of 18:1. The reduction of the ratio in the fabricated detector is theoretically interpreted by a finite-difference time-domain method on the basis of the actual grating structures. Finally, the enhancement of the extinction ratio is discussed.