Transfert radiatif | EPFL Graph Search

vignette|Températures dans un jet de disque protoplanétaire. Calcul couplé utilisant la méthode des moments. vignette|Calcul d'une avalanche électronique par la méthode de Monte-Carlo (Geant4). Le transfert radiatif (ou transfert par rayonnement) est le domaine de la physique mathématique décrivant l'interaction du rayonnement électromagnétique avec la matière, c'est-à-dire avec des particules matérielles de toutes natures (neutron, neutrino, ion, phonon...). Cette discipline permet notamment d'analyser la propagation des photons ou d'autres particules à travers un milieu gazeux, solide ou liquide. Cette propagation est affectée par les phénomènes d'émission, de réflexion, d'absorption et de diffusion qui sont traités par la physique atomique (propriétés spectroscopiques, diffusions Compton, Thomson, Mott, Møller), l'électromagnétisme (diffusion de Mie, réflexion et réfraction des interfaces) et la physique du solide (propriétés des surfaces : réflexion, émissivité). Applicatio

Plasma radiation for atmospheric entry at Titan: Emission spectroscopy measurements and numerical rebuilding

Pénélope Leyland, Raffaello Sobbia

Emission spectroscopy measurements on a plasma representative of Titan atmosphere composition were obtained in the Inductively Coupled Plasma wind tunnel facility (VKI-Minitorch) at the von Karman Institute in Belgium. Temperatures ranged from 3600 to 5000 K, pressure was fixed at 300 mbar, and the molar composition was 1.9% CH4 and 98.1% N-2. The high-pressure plasma was produced to obtain conditions close to equilibrium. In conjunction, line-by-line calculations have been carried out to assess the reliability of two distinct sets of molecular electronic transition moments, recently released, by predicting the radiative signature of high-temperature N-2-CH4 plasma. The radiative transfer problem was solved by considering the plasma plume at local thermodynamic equilibrium conditions in an axisymmetric configuration. Comparisons between the synthetic and experimental spectra demonstrated good agreement for the CN Violet and high-wavelength CN Red bands, while some discrepancies were observed for the C-2 Swan bands and low-wavelength CN Red bands.

Cambridge University Press2013

Tomography-based radiative characterisation of decomposing carbonaceous heat shield materials

Nikhil Banerji, Sophia Haussener, Pénélope Leyland

This work evaluates the changes in radiative properties of two decomposing carbonaceous porous materials, each composed of two semitransparent, homogeneous and isotropic phases. The understanding of the complex dependence of macroscopic optical behaviour on material microstructure, bulk phase properties and the wavelength of incoming radiation is paramount for modelling, design and optimisation of systems incorporating such media. Experimental and numerical techniques were combined to solve the homogenised radiative transfer equations using Monte Carlo ray tracing in the limit of geometrical optics. Effective radiative properties required by these equations were determined by Monte Carlo techniques using the exact 3D microstructures of the samples, obtained through high-resolution synchrotron computed tomography. This methodology is applied to medium density carbon phenolic and high density graphite reinforced polymer composite, each composed of semi-transparent solid and fluid phases. The extent of material decomposition is seen to affect the absorption behaviour of both samples. This effect is more obvious in the lower density carbon phenolic, where an 18% increase in absorptance is observed due to decomposition, compared to an increase of just 2% for the graphite. A library of absorption data is presented for use in continuum heat transfer modelling of similar chemically reacting macroporous carbon composites.

Pergamon-Elsevier Science Ltd2017

Decoherence and excitation-transfer mechanisms in semiconductor quantum dots

Gaetano Parascandolo

The aim of this thesis is to supply a theoretical study of the interaction mechanisms between quantum dots beyond the simple picture of macroatoms. Coulomb interaction between excitons, exciton-phonon interaction as well as radiative interaction are, in particular, considered. These mechanisms can be exploited to coherently couple quantum dots, thus being the physical tool enabling quantum information processing using quantum-dot-based logic gates. On the other hand, the same mechanisms are responsible for the decoherence of the quantum-state that prevent the storing of the quantum information. Already if considered as simple two-level systems, quantum dots are subject to mutual interaction. Quantum dots in the excited state can be considered as dipoles, and are thus coupled with each other via the dipole-dipole electrostatic interaction. This results in excitation transfer between dots over distances of a few tens of nanometers. In Chapter 2 we show that taking into account the retarded nature of the electromagnetic field results in a correction to this effect, that become a leading contribution at large distances, effectively coupling quantum dots over distances of a few hundreds of nanometers. Strong exciton-phonon-coupling in quantum dots results in a very efficient decoherence mechanism. The strongly localized polarization in an excited quantum dot can induce virtual phonon emission and reabsorption processes which act as a phase-destroying mechanism. In quantum dot molecules the decay rate of the interband polarization is almost one order of magnitude larger than in the single quantum dot case, and depends on the interdot distance. The description of this coupling mechanism is possible only beyond the marcoatom picture. In Chapter 3 we develop a model that describes the phonon-mediated interaction between quantum dots in a dot molecule, explaining the strong distance dependence of the exciton dephasing rates in terms of a matching condition between the phonon wavelength and the interdot distance, which enhances the phonon-assisted scattering from bright to dark states. The heterodyne spectral interferometry is a novel implementation of transient nonlinear spectroscopy that enables to study the transient nonlinear polarization emitted from individual localized electronic transitions, in both intensity and phase. Two-dimensional spectra obtained by means of this technique display signals that can be associated to the coherent coupling between different resonances of the system under study. This technique is theoretically modeled for the first time in Chapter 4, where a very satisfactory description of the measured spectra is provided, showing that coherent coupling between different optical transitions of a quantum system result in off-diagonal peaks of the two-dimensional excitation spectrum. Furthermore, we show that in the low intensity regime, each spectral signal can be associated to a specific pair of coupled resonances as long as the level structure of the system under study is known.

EPFL2007

Decoherence and excitation-transfer mechanisms in semiconductor quantum dots

Gaetano Parascandolo

EPFL2007