Multiple scattering of polarized light in turbid birefringent media: a Monte Carlo simulation

Transmission matrix-based Electric field Monte Carlo study and experimental validation of the propagation characteristics of Bessel beams in turbid media

A novel Transmission matrix-based Electric field Monte Carlo (TEMC) method is introduced to study the propagation characteristics of Bessel beams with different orbital angular momentum (OAM) in turbid media. As an extension to the Electric field Monte Carlo (EMC) approach, electric field transmission modes were simulated to properly evaluate light interference. Beam transmission patterns, intensity attenuation, and the degree of polarization (DOP) through turbid media of varying thickness were analyzed. It was found that the OAM plays a subtle role in transmission through turbid media, showing only a weak correlation with total transmission, the preservation of DOP, and the penetration depth. The TEMC simulation results were in excellent agreement with experiments, validating the proposed method for the study of coherence phenomenon in turbid media.

Multiple scattering of polarized light in birefringent slab media: experimental verifications and simulations

The effective scattering Mueller matrices were measured for backward and forward scattering by applying a narrow polarized light on a polyacrylamide slab gel, which was strained vertically to generate birefringence inside. Monte Carlo simulations were performed in conditions that were the same as possible. The measured and simulated matrices were simplified to the reduced ones. They agreed well in both original and reduced forms. While they approximately take reciprocal forms for backward scattering, they approximately satisfy matrix forms that correspond to a reciprocal position of the mirror image for forward scattering. The reduced matrices were factorized by the Lu-Chipman polar decomposition to obtain the polarization parameters. The polarization parameters were in good agreement between the measurement and simulation and showed characteristic features of anisotropic slab media with a birefringence axis parallel to the slab surface.

A new Monte Carlo code for light transport in biological tissue

The aim of this work was to develop an event-by-event Monte Carlo code for light transport (called MCLTmx) to identify and quantify ballistic, diffuse, and absorbed photons, as well as their interaction coordinates inside the biological tissue. The mean free path length was computed between two interactions for scattering or absorption processes, and if necessary scatter angles were calculated, until the photon disappeared or went out of region of interest. A three-layer array (air-tissue-air) was used, forming a semi-infinite sandwich. The light source was placed at (0,0,0), emitting towards (0,0,1). The input data were: refractive indices, target thickness (0.02, 0.05, 0.1, 0.5, and 1 cm), number of particle histories, and λ from which the code calculated: anisotropy, scattering, and absorption coefficients. Validation presents differences less than 0.1% compared with that reported in the literature. The MCLTmx code discriminates between ballistic and diffuse photons, and inside of biological tissue, it calculates: specular reflection, diffuse reflection, ballistics transmission, diffuse transmission and absorption, and all parameters dependent on wavelength and thickness. The MCLTmx code can be useful for light transport inside any medium by changing the parameters that describe the new medium: anisotropy, dispersion and attenuation coefficients, and refractive indices for specific wavelength.

Multiple scattering of polarized light in uniaxial turbid media with arbitrarily oriented linear birefringence

The effective scattering Mueller matrices obtained by the simulation were simplified to the reduced matrices and factorized using the Lu–Chipman polar decomposition, which afforded the polarization parameters in two dimensions. In general, the scalar retardance around the illumination point of a pencil beam shows a broad azimuthal dependence with an offset. Photons may behave quite differently under the birefringence according to their polarization state. In contrast, when the birefringence is oriented along the y -axis in the plane parallel to the surface ( x ? y ) plane, for example, the azimuthal dependence of the scalar retardance shows clear maxima along the x - and y -axes and sharp valleys between the maxima. Photons propagating in the medium probably experience the retardance in nearly the same way when they are polarized linearly and circularly. Moreover, the polarization parameters generally become nonsymmetric with respect to the plane perpendicular to both the x - y plane and the plane containing the birefringence axis, which suggests that the pathway of the lateral propagation of photons from the illumination point to the surrounding is slightly oblique upward relative to the x - y plane. These results were also compared with the case in which the birefringence axis is perpendicular to the x - y plane.

Forward scattering of polarized light from a turbid slab: theory and Monte Carlo simulations

It is proved that if reciprocity and mirror symmetry hold for single scattering by a particle, they also hold for multiple scattering in turbid slab media. Monte Carlo simulations generate a reduced effective Mueller matrix for forward scattering, which satisfies reciprocity and mirror symmetry, but satisfies only reciprocity if the medium contains chiral components. The scattering matrix was factorized by using the Lu-Chipman polar decomposition, which affords the polarization parameters as a function of the radial distance from the center. The depolarization coefficients decrease with increasing distance, whereas the scattering-induced linear diattenuation and retardance become larger in the middle-distance range. The optical rotation for a chiral medium increases with increasing distance.