Multiply-scattering Media Research Articles

Electron transport and electron density inside quasi-one-dimensional disordered conductors

We consider the problem of electron transport across a quasi-one-dimensional disordered multiply-scattering medium, and study the statistical properties of the electron density inside the system. In the physical setup that we contemplate, electrons of a given energy feed the disordered conductor from one end. The physical quantity that is mainly considered is the logarithm of the electron density, $\ln {\cal W}(x)$, since its statistical properties exhibit a self-averaging behavior. We also describe a {\em gedanken} experiment, as a possible setup to measure the electron density. We study analytically and through computer simulations the ballistic, diffusive and localized regimes. We generally find a good agreement between the two approaches. The extension of the techniques that were used in the past to find information outside the sample is done in terms of the scattering properties of the two segments that form the entire conductor on each side of the observation point. The problem is of interest in various other branches of physics, as electrodynamics and elasticity.

Twisting of Polarization of the Light Beam in a Multiply Scattering Medium

We calculate the intensity and the main mode of linear polarization, i.e., the difference in the intensities of the orthogonally polarized components, for the case of oblique incidence of a linearly polarized light beam on a disordered medium. It is assumed that multiple scattering has a sharply anisotropic character. The vector equation of radiation transfer is solved neglecting the difference in the amplitude of single scattered waves, when light becomes depolarized due to polarization twisting in the process of multiple scattering. It is shown that the angle at which the maximum of the polarization degree is observed decreases as the depth increases much slower than the angle corresponding to the intensity peak.

Imaging with nature: compressive imaging using a multiply scattering medium.

The recent theory of compressive sensing leverages upon the structure of signals to acquire them with much fewer measurements than was previously thought necessary, and certainly well below the traditional Nyquist-Shannon sampling rate. However, most implementations developed to take advantage of this framework revolve around controlling the measurements with carefully engineered material or acquisition sequences. Instead, we use the natural randomness of wave propagation through multiply scattering media as an optimal and instantaneous compressive imaging mechanism. Waves reflected from an object are detected after propagation through a well-characterized complex medium. Each local measurement thus contains global information about the object, yielding a purely analog compressive sensing method. We experimentally demonstrate the effectiveness of the proposed approach for optical imaging by using a 300-micrometer thick layer of white paint as the compressive imaging device. Scattering media are thus promising candidates for designing efficient and compact compressive imagers.

Fluorescence Lifetime Spectroscopy in Multiply Scattering Media with Dyes Exhibiting Multiexponential Decay Kinetics

To investigate fluorescence lifetime spectroscopy in tissue-like scattering, measurements of phase modulation as a function of modulation frequency were made using two fluorescent dyes exhibiting single exponential decay kinetics in a 2% intralipid solution. To experimentally simulate fluorescence multiexponential decay kinetics, we varied the concentration ratios of the two dyes, 3,3-diethylthiatricarbocyanine iodide and indocynanine green (ICG), which exhibit distinctly different lifetimes of 1.33 and 0.57 ns, respectively. The experimental results were then compared with values predicted using the optical diffusion equation incorporating 1) biexponential decay, 2) average of the biexponential decay, as well as 3) stretched exponential decay kinetic models to describe kinetics owing to independent and quenched relaxation of the two dyes. Our results show that while all kinetic models could describe phase-modulation data in nonscattering solution, when incorporated into the diffusion equation, the kinetic parameters failed to likewise predict phase-modulation data in scattering solutions. We attribute the results to the insensitivity of phase-modulation measurements in nonscattering solutions and the inaccuracy of the derived kinetic parameters. Our results suggest the high sensitivity of phase-modulation measurements in scattering solutions may provide greater opportunities for fluorescence lifetime spectroscopy.

Analytical and Experimental Aspects of Optical Property Determination of Turbid Media Using Frequency-Domain PTR

used in turbid media such as biological tissues 1-3 to study the sub-surface deposition of laser radiation, a task that may be difficult or impossible for many optical methods due to excessive scattering and absorption. In this work, the optical and thermal properties of tissue-like materials are observed using frequency-domain infrared photothermal radiometry. A three-dimensional heat conduction formulation with the use of three-dimensional optical diffusion is developed to derive a turbid frequency-domain PTR model. The agreement in the absorption and transport scattering coefficients of model phantoms is investigated. The present opto-thermal model for frequency-domain PTR may prove useful for non-contact, non-invasive, in situ measurement of optical properties of tissues and other multiply-scattering media.

Dynamics of stimulated emission from random media

The spatial and spectral evolution of emission from gain media with homogeneously distributed scatterers is modeled using a Monte Carlo simulation of the random walk of pump and emitted photons. We find a sharp transition to ultrafast, narrow-linewidth emission for a 10-ps incident pump pulse and a rapid approach to steady state for longer pulses. The threshold for this transition is raised significantly when the transverse diffusion of photons out of the excited region is substantial. The quantitative agreement of the results with recent observations of emission from random gain media lends further support to the interpretation of ``random lasers'' in terms of amplified spontaneous emission within a multiply-scattering medium.

Photon correlation and scattering: Introduction by the feature editors

This feature issue brings together fourteen papers, most of which were presented at the Eighth Topical Meeting on Photon Correlation and Scattering (Boulder, Colorado, 24-26 August 1992). Research areas represented here include dynamic light scattering; laser velocimetry; diffusing-wave spectroscopy and imaging in multiply-scattering media; fundamental analyses of measurement sensitivity and accuracy; and development of new instrumentation, procedures, and data analysis algorithms. The common theme that unites these diverse topics is the idea of exploiting the statistical properties of scattered light to probe structure or motion in complex systems.

Stochastic radiation in macroheterogeneous random optical media

The radiation regime in a multiply-scattering medium with non-uniformly-spaced absorbers is influenced by the spatial heterogeneity of the optical parameters. Random spatial fluctuations of the optical properties cause fluctuations of the radiance and light fluxes, which may result in increased transmission and distorted absorption spectra as compared to the corresponding homogeneous samples. A theoretical method for the quantitative analysis of the statistical structure of the radiation field is proposed for macroheterogeneous media with random spatial fluctuations of the optical parameters. This method is based on the derivation of averaged transport equations for the statistical moments of the radiation field parameters and involves application of an averaging procedure that is similar to that used in the theory of hydrodynamics for the derivation of the equations of turbulent fluid motion.