Pseudo-circular dichroism revealed by mid-infrared Mueller-matrix polarimetry in a non-chiral anisotropic scattering...

We demonstrate that broadband Mueller-matrix polarimetric imaging in the mid-infrared reveals a previously unexplored polarization regime in which non-chiral, strongly scattering materials exhibit pronounced pseudo-circular dichroism (PCD). Using a single optical platform, a TiO₂-dispersed PVC sheet was investigated over wavelengths from 532 nm to 3750 nm. In the visible regime, the Mueller matrix is dominated by depolarization and cannot be physically interpreted by conventional decomposition schemes. In the near-infrared, the Lu-Chipman decomposition becomes valid, and the response is governed by linear birefringence and dichroism. In contrast, in the mid-infrared region around 3750 nm, pronounced enhancement of the m₁₄ and m₄₁ elements is observed, indicating an asymmetric transmission between right- and left-circularly polarized light despite the absence of molecular chirality. To elucidate the physical origin of this PCD, we develop a physical model that incorporates molecular linear birefringence with weak linear dichroism and diffraction-induced linear diattenuation arising from flow-aligned TiO₂ microstructures. The analytical expression reveals that PCD emerges only when finite birefringence, finite diattenuation anisotropy, and misalignment of their optical axes coexist. Numerical visualizations based directly on this expression reproduce the observed dependencies on retardation and relative axis angle. These results establish mid-infrared Mueller-matrix imaging as a distinct observation regime and identify PCD as what is believed to be a new contrast mechanism for probing structural anisotropy in non-chiral, strongly scattering materials.