Mueller matrix polarimetry in the near-field of micro and nanophotonic structures

This work investigates the polarimetric behavior, with a special focus on depolarization effects, of a Ronchi-type metallic grating using the split-field finite-difference time-domain (SF-FDTD) method. The study is based on pointwise computation of Mueller matrices in both near- and far-field regimes, for both subwavelength and for suprawavelength scales, and different fill factors. In the near field, we find that local Mueller matrices often lead to covariance matrices with negative eigenvalues. Even though this is typically considered a breakdown of physical realizability, we demonstrate that they accurately encode the polarimetric transfer function of the structures. The detailed physical study of the results using Mueller matrix derived polarimetric descriptors needs to be restricted to regions where all eigenvalues remain positive, with clear trends: strong depolarization for subwavelength periods and low fill factors, while higher degrees of purity and reduced anisotropy are observed for larger periods and higher fill factors. Far-field Mueller matrices, associated with the diffraction orders, are always physically realizable and exhibit high polarimetric purity. The contrast between near- and far-field responses emphasizes the need for careful interpretation of local polarimetric data in nanostructures. These results support the use of polarimetric descriptors as valuable tools in the characterization of nanoscale optical systems.