Structurally anisotropic materials are ubiquitous in several application fields, yet their accurate optical characterization remains challenging due to the lack of general models linking their scattering coefficients to the macroscopic transport observables and the need to combine multiple measurements to retrieve their direction-dependent values. Here, we present an improved method for the experimental determination of light-transport tensor coefficients from the diffusive rates measured along all three directions, based on transient transmittance measurements and a generalized Monte Carlo model. We apply our method to the characterization of light-transport properties in two common anisotropic materials—polytetrafluoroethylene tape and paper—highlighting the magnitude of systematic deviations that are typically incurred when neglecting anisotropy.
Studying the depolarization rate of light emerging from a turbid medium holds promise for the non-invasive characterization of its single-scattering properties, with relevant application in the quality analysis of different specimens or for diagnostic purposes in the biomedical field, to name a few. However, irrespective of sample geometry, the dynamics of light depolarization takes place on a time scale of few ps, which is too fast for traditional detection methods. Here, we present experimental results on the time-domain evolution of the depolarization ratio of light that is diffusely reflected from a scattering medium, using linearly polarized fs pulses in an all-optical gating scheme. Time-resolved reflectance curves are recorded in the parallel and perpendicular polarization channels relative to the illumination beam, granting direct access to the depolarization rate. We demonstrate our experimental approach on a lipid emulsion, fitting the data with a polarized Monte Carlo simulation to retrieve the average particle size and scattering asymmetry factor using just two time-domain reflectance measurements in a semi-infinite geometry.
Structurally anisotropic materials are ubiquitous in several application fields, yet their accurate optical characterization remains challenging due our incomplete understanding of how anisotropic light transport properties arise from the microscopic scattering coefficients. In fact, even when the dynamics of light transport is directly measured, coarse simplifications are often introduced due to a lack of established theoretical models or numerical methods. Here, we apply a general Monte Carlo implementation capable of handling direction-dependent scattering to the analysis of light transport in a sample of polytetrafluoroethylene (PTFE) tape. Using only a set of transient transmittance intensity profiles, the analysis retrieves the tensor components of the diffusive rates and the scattering coefficients along all three directions, in excellent agreement with Monte Carlo simulations.
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