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Terahertz Mueller Matrix Polarimetry and Polar Decomposition

Mueller matrix imaging, as one of the polarimetric imaging modalities, is an effective tool to achieve imaging enhancement as well as explore fine details of the imaged sample at microwave and optical frequencies. However, little work has been reported at terahertz (THz) frequencies, which could provide an exclusive insight in exploring fine details of biological specimens for their sensitivity to THz absorption due to their various water contents. This article presents a 0.3-THz polarimetric system for Mueller matrix imaging measurements both in reflection and transmission modes. Two experiments are designed to demonstrate the capability of Mueller matrix theory and its polar decomposition in imaging enhancement and properties characterization. The structural similarity index (SSIM) is employed to serve as the metric for image quality assessment, by which the performance of the Mueller matrix imaging is evaluated. The SSIM values of the Mueller matrix could reach as high as 0.9–0.98 for the leaf sample and 0.9 for the indium tin oxide (ITO) target, respectively, whereas that of the raw polarization images could only stabilize at 0.7–0.9 and 0.2–0.4. Furthermore, the SSIM value of the decomposition parameters for the ITO target could reach nearly 1, the maximum value. The imaging results both in visual and analytical perspectives demonstrate successful system integration at THz band. The work paves the way for our exploration of biomedical imaging of tissues and cells at higher THz frequencies in the immediate future.