S. Menon

Scaling of light scattering with density of scatterers

We inject a laser beam into a tank filled with a milk-water emulsion and measure the intensity distribution of the scattered light. As we change the concentration of the milk, we observe a nontrivial change in the light intensity as a function of the detector position. We analyze the light on and parallel to the input beam direction, as well as the scattered light in the transverse direction. The nonmonotonic scaling of the intensity as a function of the concentration and the position is also predicted by Monte Carlo simulations. With a doubling of the concentration, the detected light along the optical axis decreases globally, whereas the reflected light decreases or increases depending on the location of the detector.

Generalized diffusion solution for light scattering from anisotropic sources.

The traditional diffusion theory, often used for isotropic sources, becomes inaccurate at short source-detector spacings and cannot be applied to media with large absorption or with small scattering strengths. We show that for any type of source anisotropy, a Greens-function-based procedure can remove these limitations. The accuracy of the new approach is examined through a comparison with the numerical solution to the radiative transfer equation.

High-frequency limit of photon density waves in turbid media

We propose an iterative scheme to compute quasi-analytically the laser irradiance transmitted through a turbid media illuminated by high-frequency intensity-modulated light. We compare the spatial distribution of the transmitted signal obtained from iterative calculations with that obtained from random walk simulations and from the traditional diffusion theory. In the regime where the diffusion theory breaks down for a highly modulated source, only the ballistic and quasi-ballistic photons survive. This regime could become accessible for interesting imaging applications as the beam width of the AC signal of the transmitted light decreases with an increasing frequency.

Velocity half-sphere model for multiple scattering in a semi-infinite medium

We show how the velocity half-sphere model [S. Menon, Q. Su, and R. Grobe, Phys. Rev. E 72, 041910 (2005)] recently introduced to predict the propagation of light for an infinite turbid medium can be extended to account for the emission of multiply scattered light for a geometry with a planar boundary. A comparison with exact solutions obtained from Monte Carlo simulations suggests that this approach can improve the predictions of the usual diffusion theory for both isotropic and highly forward scattering media with reflecting interfaces.

Imaging in turbid media with intensity-modulated lasers

We describe the formation of a narrow beam for intensity-modulated electromagnetic radiation propagating through highly scattering materials. We propose to use this beam to reconstruct images, similar to X-ray back-projection techniques. For sufficiently high modulation frequency, the photon density wave is primarily carried by photons that suffer small or no large-angle scattering, which gives rise to the beam’s narrow divergence. The beam-narrowing concept is supported by large-scale numerical simulations to examine the quality of the imaging.

Point scatterer phase approximation for electromagnetic and intensity waves in random media

The validity of a point scatterer approximation for the phases of electromagnetic and also intensity waves is examined for multilayered random media using a transfer matrix approach. In this study the phase of the transmitted light for each scatterer is approximated by the phase characteristic of an infinitely narrow layer with an infinitely large index of refraction. The approximated phase depends only on the transmission coefficient and permits a reduction of the number of microscopic variables required for such a system.

Split operator solution of the time-dependent Maxwell's equations for random scatterers

We discuss how a spectral-domain method in combination with a split-operator technique can be used to calculate exact solutions of the time-dependent Maxwells equations. We apply this technique to study the propagation of a light pulse through an inhomogeneous medium consisting of multiple random scatterers. We investigate the validity of the Boltzmann equation by directly comparing its solution with the ensemble averaged Maxwell solution.

Numerical solution techniques to the time-dependent Maxwell equations for highly scattering media

We discuss how a spectral-domain method in combination with a split-operator technique can be used to calculate exact solutions of the time-dependent Maxwell equations. We apply this technique to study the propagation of a light pulse through an inhomogeneous medium consisting of practically arbitrarily shaped dielectric and metallic materials.