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.
In many biological tissues such as muscles, dental enamel and mucosa that exhibit macroscopic and/or microscopic spatially anisotropic structures the nature of light scattering becomes anisotropic. This well-known property of tissues is usually neglected, which is rooted in the fact that the available open-source numerical solutions to the radiative transfer equation based on the stochastic Monte Carlo (MC) method do not allow simulations with anisotropic optical properties. In this contribution, we present an extension to our massively parallel PyXOpto (https://github.com/xopto/pyxopto) simulation engine that enables highly efficient and user-friendly MC simulations for layered or voxelated sample geometries with anisotropic scattering properties, both in the steady state and time-resolved domain.
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.
Usually, in biomedical optics, the average photon fluence rate, evaluated in a subvolume of a propagating medium, is obtained by Monte Carlo simulations by calculating the power deposited by photons absorbed in the subvolume. We propose an alternative method based on evaluating the average path length traveled by all photons injected within the subvolume. Application examples are given. This method also works for a zero absorption coefficient and for a nonconstant spatial distribution of the absorption coefficient within the subvolume. The proposed approach is a re-visitation of a well-known method applied to nuclear and radiation physics. The results obtained show that a potential advantage of the proposed method is that it can improve the convergence of Monte Carlo simulations. Indeed, when calculating the fluence in a region of interest with the proposed method, all photons passing through the region are considered. Whereas with the traditional approach, only absorbed" photons are considered. In the latter case, this can produce a poorer Monte Carlo statistic for the same number of photons launched.
Significance: Most radiative transport problems in turbid media are typically associated with mm or cm scales, leading to typical time scales in the range of hundreds of ps or more. In certain cases, however, much thinner layers can also be relevant, which can dramatically alter the overall transport properties of a scattering medium. Studying scattering in these thin layers requires ultrafast detection techniques and adaptations to the common Monte Carlo (MC) approach.
Aim: We aim to discuss a few relevant aspects for the simulation of light transport in thin scattering membranes, and compare the obtained numerical results with experimental measurements based on an all-optical gating technique.
Approach: A thin membrane with controlled scattering properties based on polymer-dispersed TiO2 nanoparticles is fabricated for experimental validation. Transmittance measurements are compared against a custom open-source MC implementation including specific pulse profiles for tightly focused femtosecond laser pulses.
Results: Experimental transmittance data of ultrafast pulses through a thin scattering sample are compared with MC simulations in the spatiotemporal domain to retrieve its scattering properties. The results show good agreement also at short distances and time scales.
Conclusions: When simulating light transport in scattering membranes with thicknesses in the orders of tens of micrometer, care has to be taken when describing the temporal, spatial, and divergence profiles of the source term, as well as the possible truncation of step length distributions, which could be introduced by simple strategies for the generation of random exponentially distributed variables.
Among the natural white colored photonics structures, a bio-system has become of great interest in the field of disordered optical media: the scale of the white beetle Chyphochilus. Despite its low thickness, on average 7 μm, and low refractive index, this beetle exhibits extreme high brightness and unique whiteness. These properties arise from the interaction of light with a complex network of chitin nano filaments embedded in the interior of the scales. As it’s been recently claimed, this could be a consequence of the peculiar morphology of the filaments network that, by means of high filling fraction (0.61) and structural anisotropy, optimizes the multiple scattering of light. We therefore performed a numerical analysis on the structural properties of the chitin network in order to understand their role in the enhancement of the scale scattering intensity. Modeling the filaments as interconnected rod shaped scattering centers, we numerically generated the spatial coordinates of the network components. Controlling the quantities that are claimed to play a fundamental role in the brightness and whiteness properties of the investigated system (filling fraction and average rods orientation, i.e. the anisotropy of the ensemble of scattering centers), we obtained a set of customized random networks. FDTD simulations of light transport have been performed on these systems, observing high reflectance for all the visible frequencies and proving the implemented algorithm to numerically generate the structures is suitable to investigate the dependence of reflectance by anisotropy.
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