Recent advances of the biomedical polarimetry have clearly demonstrated that circularly polarized light (CPL) can be effectively used for overall characterization of biological tissues with optical anisotropy,^{1 – 3} including detection of the senile Alzheimer’s plaques^4,5 and quantitative evaluation of the cervical intraepithelial neoplasia.^6,7 Proper exploration of the CPL-tissue interaction requires accurate self-consistent descriptive simulation tools.^1,8,9 Monte Carlo (MC) based approaches are widely recognized as efficient tools for analyzing light scattering by biological tissues and turbid medium.^{10 – 14} In biophotonics, MC methods, such as MCML,¹⁵ created by L. Wang and S. Jacques, were originally designed to simulate scalar light transport within turbid scattering medium^16,17 and were fundamentally relying on the radiative transfer equation (RTE).^{18 – 20} As significant role of polarized light in extending diagnostic capabilities of biomedical tools became apparent,^21,22 MC methods evolved accordingly resulting in many practical and popular tools particularly developed by Ramella-Roman, Prahl, and Jacques,^23,24 Hielscher,^25,26 Wang,²⁷ and Xu.²⁸ Fundamental ground for these polarized MC approaches was established by the vector radiative transfer equation (VRTE), which represents a system of equations for each Stokes parameter and can be rigorously derived from the Maxwell electromagnetic theory.^{29 – 31} At the same time, an approach based on the iterative solution to Bethe–Salpeter (BS) equation ^{19,32 – 34} utilizing Jones vector formalism has been demonstrated to be effective for polarization tracking of MC-photons within turbid tissue-like medium and simulation of coherent backscattering.^{13,14,35 – 39} Recently, it has been shown on the fundamental level that VRTE- and BS-based approaches are equivalent under certain conditions.⁴⁰ Advantages of the BS-based approach involve a direct relation to the analytic Milne solution and intuitive physical interpretation of the multiple scattering process via ladder diagrams.

The minimum histological criterion for the diagnostics of Alzheimer’s disease (AD) in tissue is the presence of senile plaques and neurofibrillary tangles in specific brain locations. The routine procedure of morphological analysis implies time-consuming and laborious steps including sectioning and staining of formalin-fixed paraffin-embedded (FFPE) tissue. We developed a multispectral Stokes polarimetric imaging approach that allows characterization of FFPE brain tissue samples to discern the stages of AD progression without sectioning and staining the tissue. The Stokes polarimetry approach is highly sensitive to structural alterations of brain tissue, particularly to the changes in light scattering and birefringence. We present the results of the label-free non-destructive screening of FFPE mouse brain tissue and show several polarization metrics that demonstrate statistically significant differences for tissues at different stages of AD.

We consider the origin of unpolarized light resulting from the backscattering of circularly polarized light by a random turbid tissuelike disperse medium. We reveal the dynamics of the backscattered fraction of unpolarized light, disclosing its meticulous decomposition into two rigorously polarized components characterized by opposing helicities, with fully defined polarization states. Concurrently, their superposition, driven by multiple scattering within the medium, leads to the appearance of a fraction of linear polarization. We emphasize that the in-depth binding of circular polarization memory of light when the helicity flips occurs within the scattering medium, meaning the conservation of spin angular momentum. We anticipate that the results obtained hold significant implications for future studies, particularly in the field of tissue polarimetry and light vortices.

SignificanceThe study of the effect of aging on the optical properties of biological tissues, in particular polarization, is important in the development of new diagnostic approaches.AimThis work aims to provide a comprehensive analysis of the factors and mechanisms that contribute to the alteration of skin polarization properties caused by aging, using polarization-sensitive hyperspectral imaging measurements and Monte Carlo simulation.ApproachOur investigation involved both experimental studies of in vivo human skin of volunteers of different ages and computational modeling that accounted for changes in the absorption and scattering properties of the skin model. Specifically, we analyzed alterations in the degree of linear polarization (DOLP) to better understand the impact of aging on skin polarization properties.ResultsA statistically significant increase in the DOLP was found for the elderly group. At the same time, there was no correlation between changes in polarization and the calculated blood volume fraction parameter for different ages. According to the simulation results, it was also found that a change in the scattering properties of biological tissues has a more significant effect on the change in polarizing light compared to the change in absorption.ConclusionsThe results of the work prove that the sensitivity of polarization imaging to age- or pathological-related skin changes may be primarily due to changes in scattering, which in turn is associated with changes in the collagen fibers of the dermis. The proposed technique shows promise for in vivo non-invasive real-time assessment of age-associated skin changes and can also be extended to monitor changes associated with the development of age-related pathologies.

A three dimensional problem of plane wave diffraction on a plasmonic nanoparticle is considered, with account for the nonlocal effect. A solution is constructed on the basis of a modified computationa