Spectral-Polarization Sensing in Visible Wavelengths Enabled by Nanostructural Encoders

Abstract

A challenge for spectral-polarimetric sensors is their size. Most spectropolarimeters today are benchtop instruments. To address this challenge, we introduce a chip-scale spectropolarimeter design based on nanoscale GaN spectral and polarization encoders and photodetectors. Our proof-of-concept experiments validated the feasibility of measuring an optical signal’s spectrum and polarization in the visible wavelengths, including all four of Stokes parameters. Unlike a conventional spectropolarimeter in which optics completely resolve individual spectral and polarization components, the encoders create slight differences in them. A reconstruction algorithm, e.g., non-negative least-squares optimization, together with proper regularizers, then recovers the spectrum and polarization using a priori response functions of those encoders. Using nanoscale GaN light-emitting diodes with elliptical cross sections, this work demonstrates the recovery of full Stokes parameters in the visible wavelength range, including the degree of polarization for a partially polarized signal. A 1.22° accuracy in the angle of polarization was shown. The vertical optical path enables a high-density integration for imaging spectropolarimetry applications. If the polarization is independent or only a weak function of the wavelength, a single pair of elliptically shaped encoders was shown to suffice to reconstruct the angle of polarization in the range of 0 and 90°.