Characterizing thermal emission polarization with a mid-wave infrared broadband polarization state analyzer between 2.5 and 5 μm

Recent progress in the design and fabrication of thermal metasurfaces allows a broad control of the properties of light emission, including its polarization state. Stokes polarimetry is a key approach to accurately characterize partially polarized light. The quality of a Stokes polarimeter made of retarders and polarizers can be evaluated by use of metrics such as the equally weighted variance or the condition number of the matrix representing the polarimeter. Although specific instrument configurations are used to maximize polarimeter performance at a given wavelength, such optimal solutions are not spectrally robust because of the wavelength dependence of retardance. This becomes an issue in characterizing broadband thermal sources in the infrared. We report a Stokes polarimeter making use of five polarization analysis states and consisting of two simple and common optical elements—a crystalline waveplate and a linear polarizer. We combine this setup with a Fourier transform infrared spectrometer to measure accurately in a single set of acquisitions without requiring any spectral filtering, and to measure the polarization state with accuracy over a broad range of wavelengths. Such a Stokes polarimeter allows for close to optimal noise in the data reduction process in the mid-wave infrared spectral range from 2.5 to 5 μm.