La grande surface développée du silicium poreux fait de lui un hôte potentiel pour l'incorporation de molécules organiques. Les nanocomposites hybrides à base de molécules conjuguées luminescentes pourraient se prêter à des applications en optoélectronique.<br />L'étude comprend deux parties : la première est consacrée à l'étude morphologique et optique des couches minces et des membranes auto-supportées de silicium poreux fabriquées au laboratoire en utilisant la microscopie électronique, l'ellipsométrie spectroscopique et l'absorption. L'analyse microstructurale des couches poreuses par diffusion Raman nous a permis d'estimer la distribution de tailles des nanocristallites via le modèle de confinement des phonons et de confirmer l'absence de porteurs de charges libres. Une étude de Raman polarisé sur des membranes poreuses libres permet de sonder les inhomogénéités de propagation de la lumière dans ce milieu.<br />La seconde partie présente les études concernant l'imprégnation de molécules fluorescentes dans les pores. La photoluminescence donne un moyen de vérifier l'efficacité de l'incorporation de molécule de Rodhamine R6G et son homogénéité. L'excitation sélective permet une approche des transferts d'énergie entre les deux matériaux. La photoluminescence résolue en temps montre que la présence de la R6G crée de nouveaux canaux de désexcitation non radiative.<br />Le THD est incorporé dans des membranes libres rendues organophiles, puis polymérise spontanément in situ en son poly-Diacétylène. La variation angulaire de la photoluminescence et du Raman témoignent de la présence de chaînes de polymères dont le degré d'orientation est compatible avec une croissance le long des pores.

Une variété horosphérique est une variété algébrique normale dans laquelle un groupe algébrique réductif opère avec une orbite ouverte fibrée en tores sur une variété de drapeaux. En particulier, les variétés toriques et les variétés de drapeaux sont horosphériques. Dans cet article, on classifie les variétés horosphériques de Fano en termes de certains polytopes rationnels qui généralisent les polytopes réflexifs considérés par V. Batyrev. Puis on obtient une majoration du degré des variétés horosphériques lisses de Fano, analogue à celle donnée par O. Debarre dans le cas torique. On étend un résultat récent de C. Casagrande: les variétés horosphériques Q-factorielles de Fano ont leur nombre de Picard majoré par deux fois la dimension. On donne aussi de nombreux exemples en rang 2.

Strongly coupled plasmon-exciton hybrid nanostructures are fabricated and their optical properties are studied. The plasmonic and excitonic systems are gold nanoshells and J-aggregates, respectively. Gold nanoshells are tunable plasmonic core-shell nanoparticles which can sustain distinct dipole and quadrupole plasmons with resonant energies dependent on core-size/shell-thickness ratio. J-aggregates are organic semiconducting material with excitons that possess very high oscillator strength making them suitable for coherent interaction with other kinds of excitations. The J-aggregates are formed on the surface of the nanoshells when a water/ethanol (50:50) solution of the dye molecules (2,2'-dimethyl-8-phenyl-5,6,5',6'-dibenzothiacarbocyanine chloride) is added to an aqueous solution of nanoshells. These nanoshell-J-aggregate complexes exhibit coherent coupling between localized plasmons of the nanoshell and excitons of the molecular J-aggregates. Coherent coupling strengths of 120 meV and 100 meV have been measured for dipole and quadrupole plasmon interactions with excitons, respectively. Femtosecond time-resolved transmission spectroscopy studies are carried out in order to understand the possible sources of optical nonlinearities in the nanoshell-J-aggregate hybrid. Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate. An additional, transient Fano-shaped modulation within the Fano dip is also observable. The transient behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon.

Quantum coherence and interference(QCI) effects have been studied for decades and are widely exploited in many areas. For media with QCI effect, the optical properties can change drastically, which leads to many interesting effects, such as coherent population trapping, electromagnetically induced transparency(EIT), lasing without population inversion(LWI) and so on. We have theoretically studied the pulsed regime of EIT. In particular, simulations of propagation of gaussian and 0 - pi co-propagating laser pulses in a medium consisting of 3-level Lambda-atoms have been performed. It has been found that, even at the two-photon resonance, the length of propagation for the 0 - pi pulses is much smaller than that for the Gaussian probe pulses. We explained such a behavior using the dark and bright basis and the dressed state basis. Some possible applications are discussed. We also investigated the collision-induced coherence of two decay channels along two optical transitions. Quantum interference will suppress the spontaneous emission. The degree of this suppression is measured by the branch ratio of these two transitions. Our preliminary calculations show that a significant decrease of the branching ratio with increase of electron densities is reproduced in the theory. We have developed a new variant of Raman spectroscopy with shaped femtosecond pulses. It has several advantages to be applied in multiscatterd media. It is based on change of the spectra of femtopulses due to Raman scattering (stimulated or coherent). The technique can be used for a broad range of applications from atomic and molecular optical and IR spectroscopy to spore detection and tissue microscopy. Finally, we have shown that Fano interference in the decay channels of three levels system can lead to considerably different absorption and emission profiles. We found that a coherence can be built up in the ground state doublet whose strength depends on a coupling parameter that arises from Fano interference. This can in principle lead to breaking of the detail balance between the absorption and emission processes in atomic systems.

Une variété horosphérique est une variété algébrique normale dans laquelle un groupe algébrique réductif opère avec une orbite ouverte fibrée en tores sur une variété de drapeaux. La dimension de ces tores est appelée le rang de la variété horosphérique. En particulier, les variétés toriques et les variétés de drapeaux sont horosphériques. Dans cette thèse, on classifie les variétés horosphériques de Fano en termes de certains polytopes rationnels qui généralisent les polytopes réflexifs considérés par V.Batyrev. Puis on obtient une majoration du degré des variétés horosphériques lisses de Fano, analogue à celle donnée par O.Debarre dans le cas torique. On étend un résultat récent de C.Casagrande : les variétés horosphériques Q-factorielles de Fano ont leur nombre de Picard majoré par deux fois la dimension. On donne aussi de nombreux exemples en rang 2.

In this thesis we investigate experimentally certain aspects of the interaction of terahertz (THz) radiation with intersubband transitions and excitonic transitions in semiconductor quantum wells. The first part deals with a more fundamental view on an intersubband transition in a symmetric, undoped GaAs/AlGaAs multiple quantum well. After optical excitation of carriers, the considered electronic conduction intersubband transition is probed in the low-intensity linear regime using broadband THz pulses. These pulses are detected via field-resolved electro-optic sampling. While the sample’s terahertz absorption shows the expected single peak of the resonant intersubband transition, the differential transmission spectra, i.e. the photoexcitation-induced changes in transmission, display strong Fano signatures. On the basis of a microscopic theory, we show that they originate from a phase sensitive superposition of THz current and ponderomotive current. The latter one results from the wiggling motion of carriers induced by the accelerating THz field. Our findings demonstrate for the first time that the ponderomotive contribution has to be taken into account also at the lowest THz intensities. The following issues consider the interaction with THz pulses of higher intensity from the free-electron laser (FEL) of the Forschungszentrum Dresden-Rossendorf. In one experiment we investigate efficient second order sideband generation in the GaAs/AlGaAs multiple quantum well mentioned above. To this end a near-infrared laser tuned to excitonic interband transitions is mixed inside the sample with the inplane polarized FEL beam to create the sum- and difference-frequencies between them. We compare the sideband efficiencies for the THz beam tuned to the interexcitonic heavy-hole light-hole transition and to the intraexcitonic heavy-hole 1s-2p transition. In the latter case we achieve a ten times higher n=+2 low-temperature efficiency around 0.1%. This value is comparable to previous studies in the literature, but our approach involves different transitions in a much simpler geometry. At room temperature the efficiency drops only by a factor of 7 for low THz powers. The last part of this thesis addresses another fundamental quantum-mechanical phenomenon: the splitting of an absorption line in a strong THz field. In the same abovementioned quantum well sample the FEL wavelength is tuned near the intraexcitonic 1s-2p heavy-hole transition. The THz radiation induces a power-dependent splitting of the heavy-hole 1s exciton absorption line which manifests itself in the transmitted spectrum of a broadband near-infrared probe beam. The FEL-wavelength-dependent strength of this so-called Autler-Townes splitting is discussed on the basis of a simple two-level model.

Scintillation based gamma-ray detectors are widely used in medical imaging, high-energy physics, astronomy and national security. Scintillation gamma-ray detectors are field-tested, relatively inexpensive, and have good detection efficiency. Semi-conductor detectors are gaining popularity because of their superior capability to resolve gamma-ray energies. However, they are relatively hard to manufacture and therefore, at this time, not available in as large formats and much more expensive than scintillation gamma-ray detectors. Scintillation gamma-ray detectors consist of: a scintillator, a material that emits optical (scintillation) photons when it interacts with ionization radiation, and an optical detector that detects the emitted scintillation photons and converts them into an electrical signal. Compared to semiconductor gamma-ray detectors, scintillation gamma-ray detectors have relatively poor capability to resolve gamma-ray energies. This is in large part attributed to the "statistical limit" on the number of scintillation photons. The origin of this statistical limit is the assumption that scintillation photons are either Poisson distributed or super-Poisson distributed. This statistical limit is often defined by the Fano factor. The Fano factor of an integer-valued random process is defined as the ratio of its variance to its mean. Therefore, a Poisson process has a Fano factor of one. The classical theory of light limits the Fano factor of the number of photons to a value greater than or equal to one (Poisson case). However, the quantum theory of light allows for Fano factors to be less than one. We used two methods to look at the correlations between two detectors looking at same scintillation pulse to estimate the Fano factor of the scintillation photons. The relationship between the Fano factor and the correlation between the integral of the two signals detected was analytically derived, and the Fano factor was estimated using the measurements for SrI₂:Eu, YAP:Ce and CsI:Na. We also found an empirical relationship between the Fano factor and the covariance as a function of time between two detectors looking at the same scintillation pulse. This empirical model was used to estimate the Fano factor of LaBr₃:Ce and YAP:Ce using the experimentally measured timing-covariance. The estimates of the Fano factor from the time-covariance results were consistent with the estimates of the correlation between the integral signals. We found scintillation light from some scintillators to be sub-Poisson. For the same mean number of total scintillation photons, sub-Poisson light has lower noise. We then conducted a simulation study to investigate whether this low-noise sub-Poisson light can be used to improve spatial resolution. We calculated the Cramér-Rao bound for different detector geometries, position of interactions and Fano factors. The Cramér-Rao calculations were verified by generating simulated data and estimating the variance of the maximum likelihood estimator. We found that the Fano factor has no impact on the spatial resolution in gamma-ray imaging systems.

這篇論文裡，我們研究熱帶環面曲體，尤其是熱帶環面法諾曲體。如同古典代數幾何裡的情況一樣，要建構熱帶環面曲體，我們先從扇型開始建構。然而在某些結構裡沒辦法有熱帶化的對應，因此我們需要選一個適當的定義，這個定義必需可看成是古典情況類推而來的。在我們的論文中，使用我們認為合適的定義，計算所有平滑二維熱帶環面法諾曲體的情況，結果也證實非常類似古典的情形。 / In this thesis, we survey and study tropical toric varieties with focus on tropical toric Fano varieties. To construct tropical toric varieties, we start with fans, just like the situation in classical algebraic geometry. However, some constructions does not make sense in tropical settings. Therefore, we need to choose a reasonable definition which give an analogue of a classical toric variety. In the end of this paper, we use the definition we choose, and explicitly calculate all smooth two-dimensional tropical toric Fano varieties which we found are very similar to classical cases.

abstract: Nuclear proliferation concerns have resulted in a desire for radiation detectors with superior energy resolution. In this dissertation a Monte Carlo code is developed for calculating energy resolution in gamma-ray detector materials. The effects of basic material properties such as the bandgap and plasmon resonance energy are studied using a model for inelastic electron scattering based on electron energy-loss spectra. From a simplified "toy model" for a generic material, energy resolution is found to oscillate as the plasmon resonance energy is increased, and energy resolution can also depend on the valence band width. By incorporating the model developed here as an extension of the radiation transport code Penelope, photon processes are also included. The enhanced version of Penelope is used to calculate the Fano factor and average electron-hole pair energy in semiconductors silicon, gallium arsenide, zinc telluride, and scintillators cerium fluoride and lutetium oxyorthosilicate (LSO). If the effects of the valence band density-of-states and phonon scattering are removed, the calculated energy-resolution for these materials is fairly close to that for a toy model with a uniform electron energy-loss probability density function. This implies that the details of the electron cascade may in some cases have only a marginal effect on energy resolution. / Dissertation/Thesis / Ph.D. Physics 2011

An algebraic variety is called rationally connected if two generic points can be connected by a curve isomorphic to the projective line. The output of the minimal model program applied to rationally connected variety is variety admitting Mori fiber spaces over a rationally connected base. In this thesis we study the birational geometry of a particular class of rationally connected Mori fiber spaces: Fano fibrations over the projective line. We construct examples of Fano fibrations with a unique Mori fiber space in their birational classes. We prove that these examples are not birational to varieties of Fano type, thus answering the question of Cascini and Gongyo. That is we prove that the classes of rationally connected varieties and varieties of Fano type are not birationally equivalent. To construct the examples we use the techniques of birational rigidity. A Mori fiber space is called birationally rigid if there is a unique Mori fiber space structure in its birational class. The birational rigidity of smooth varieties admitting a del Pezzo fibration of degrees 1 and 2 is a well studied question. Unfortunately it is not enough to study smooth del Pezzo fibrations as there are fibrations which do not have smooth or even smoothable minimal models. In the case of fibrations of degree 2 we know that there is a minimal model with 2-Gorenstein singularities. These singularities are degenerations of the simplest terminal quotient singularity: singular points of the type 1/2(1,1,1). We give first examples of birationally rigid del Pezzo fibrations with 2-Gorenstein singularities. We then apply this result to study finite subgroups of the Cremona group of rank three. We then study the birational geometry of Fano fibrations from a different side. Using the reduction to characteristic 2 method we prove that double covers of Pn-bundles over Pm branched over a divisor of sufficiently high degree are not stably rational. For a del Pezzo fibration Y→P1 of degree 2 such that X is smooth there is a double cover Y→ X, where X is a P2-bundle over P1. In this case a stronger result holds: a very general Y with Pic(Y)≅Z⊕Z is not stably rational. We discuss the proof of this statement.