Reciprocity relation for the vector radiative transport equation and its application to diffuse optical tomography with polarized light

We derive a reciprocity relation for vector radiative transport equation (vRTE) that describes propagation of polarized light in multiple-scattering media. We then show how this result, together with translational invariance of a plane-parallel sample, can be used to compute efficiently the sensitivity kernel of diffuse optical tomography (DOT) by Monte Carlo simulations. Numerical examples of polarization-selective sensitivity kernels thus computed are given.Comment: 5 pages, 3 figure

Electromagnetic scattering with the GDT-matrix method: an application to irregular ice particles in cirrus

The study describes a new method to calculate electromagnetic scattering from small, arbitrary shaped particles. It is called the Green's dyadic technique for the transition operator (brie y the GDT-matrix). First, the method is introduced from the general theory of scattering (transition operator and Dyson equation) and then it is compared with existing electromagnetic scattering models. Second, the model is used to characterize small ice particles and to interpret single-particle scattering measurements made with the Small Ice Detector instrument (SID). In particular, the study is focused on small irregular particles with fractal shapes. We apply the Gaussian random sphere model in order to give an estimation of the particle roughness. The direct comparison of the forward scattered intensity as measured by SID and simulated by the GDT-matrix model leads to a retrieval of the size and shape of the scatterers. Third, the single particle results are statistically averaged to calculate many-particle optical properties in order to evaluate radiances through the radiative transfer Monte Carlo model McArtim. Further, the simulated radiances in near-infrared (NIR) are compared with measurements made during the NASA-ATTREX project on board the research unmanned aircraft Global Hawk. These comparisons lead to a remote retrieval of the thermodynamic phase of the constituent cloud particles and provide insights on the surface of the scatterers, i.e. rough or smooth

Cloud chamber experiments on the origin of ice crystal complexity in cirrus clouds

This is an open access article, made available under the terms of the Creative Commons attribution license CC BY 3.0 https://creativecommons.org/licenses/by/3.0/This study reports on the origin of ice crystal complexity and its influence on the angular light scattering properties of cirrus clouds. Cloud simulation experiments were conducted at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber of the Karlsruhe Institute of Technology (KIT). A new experimental procedure was applied to grow and sublimate ice particles at defined super- and subsaturated ice conditions and for temperatures in the −40 to −60 °C range. The experiments were performed for ice clouds generated via homogeneous and heterogeneous initial nucleation. Ice crystal complexity was deduced from measurements of spatially resolved single particle light scattering patterns by the latest version of the Small Ice Detector (SID-3). It was found that a high ice crystal complexity is dominating the microphysics of the simulated clouds and the degree of this complexity is dependent on the available water vapour during the crystal growth. Indications were found that the crystal complexity is influenced by unfrozen H2SO4/H2O residuals in the case of homogeneous initial ice nucleation. Angular light scattering functions of the simulated ice clouds were measured by the two currently available airborne polar nephelometers; the Polar Nephelometer (PN) probe of LaMP and the Particle Habit Imaging and Polar Scattering (PHIPS-HALO) probe of KIT. The measured scattering functions are featureless and flat in the side- and backward scattering directions resulting in low asymmetry parameters g around 0.78. It was found that these functions have a rather low sensitivity to the crystal complexity for ice clouds that were grown under typical atmospheric conditions. These results have implications for the microphysical properties of cirrus clouds and for the radiative transfer through these clouds.Peer reviewedFinal Published versio

Quasi-spherical ice in convective clouds

Homogeneous freezing of supercooled droplets occurs in convective systems in low and midlatitudes. This droplet-freezing process leads to the formation of a large amount of small ice particles, so-called frozen droplets, that are transported to the upper parts of anvil outflows, where they can influence the cloud radiative properties. However, the detailed microphysics and, thus, the scattering properties of these small ice particles are highly uncertain. Here, the link between the microphysical and optical properties of frozen droplets is investigated in cloud chamber experiments, where the frozen droplets were formed, grown, and sublimated under controlled conditions. It was found that frozen droplets developed a high degree of small-scale complexity after their initial formation and subsequent growth. During sublimation, the small-scale complexity disappeared, releasing a smooth and near-spherical ice particle. Angular light scattering and depolarization measurements confirmed that these sublimating frozen droplets scattered light similar to spherical particles: that is, they had angular light-scattering properties similar to water droplets. The knowledge gained from this laboratory study was applied to two case studies of aircraft measurements in midlatitude and tropical convective systems. The in situ aircraft measurements confirmed that the microphysics of frozen droplets is dependent on the humidity conditions they are exposed to (growth or sublimation). The existence of optically spherical frozen droplets can be important for the radiative properties of detraining convective outflows.Peer reviewe

Design of a reflectionless optical amplifier through broken-supersymmetry

A very interesting class of metamaterials is characterized by supersymmetry (SUSY). What makes SUSY very attractive for the design of new optical devices is the possibility to define different spatial refractive index distributions (superpartners) having the same scattering spectra (angularly and spectrally). In this study we explore the possibilities offered by the generation of superpartners of vacuum, being reflectionless and possessing unit transmission by definition. In particular, broken-SUSY is used to define a reflectionless active cavity capable of amplifying electromagnetic radiation in the visible. The approach is analytical through the use of the Darboux transform (a type of supersymmetric transformation) for the generation of the optical potential and the calculation of the field, while the transmission/reflection spectra evaluation is done with the Transfer Matrix method. Interestingly, we show that the Darboux transform allows to define 1D materials that are reflectionless for a continuum of frequencies. Moreover, the proposed device behaves as a dynamic optical filter amplifying radiation arriving at large angles while for other directions is almost completely transparent. Thus, simply by rotation different functionalities can be obtained. In addition, the active filter is reflectionless for all wavelengths and angles of incidence

A model for full-field optical coherence tomography in scattering media

International audienceWe develop a model of full-field optical coherence tomography (FF-OCT) that includes a description of partial temporal and spatial coherence, together with a mean-field scattering theory going beyond the Born approximation. Based on explicit expressions of the FF-OCT signal, we discuss essential features of FF-OCT imaging, such as the influence of partial coherence on the optical transfer function, and on the decay of the signal with depth. We derive the conditions under which the spatially averaged signal exhibits a pure exponential decay, providing a clear frame for the use of the Beer-Lambert law for quantitative measurements of the extinction length in scattering media

A new Monte Carlo simulation package for light transport in biological tissues and an application to detailed analysis of the diffusion approximation accuracy near the boundaries

International audienceWe present a new Monte Carlo computational package for solving the radiative transport equation. The package is applied for numerical evaluation of the validity of the diffusion approximation for transmission and back-reflection of an incident narrowly-collimated laser beam

Reciprocity relations in 3D vector radiative transport applied to diffuse optical tomography

International audienceWe describe a reciprocity relation for polarized radiative transport between arbitrarily positioned sources and detectors separated by a scattering medium. Applications to polarized Diffuse Optical Tomography are shown which allow for efficient computation of the sensitivity kernel

Reciprocity in vector radiative transport and the sensitivity of probing regions to various polarization imaging channels

International audienc