Anomalous photon diffusion in atomic vapors

The multiple scattering of photons in a hot, resonant, atomic vapor is investigated and shown to exhibit a L\'evy Flight-like behavior. Monte Carlo simulations give insights into the frequency redistribution process that originates the long steps characteristic of this class of random walk phenomena

A cold-atom random laser

Conventional lasers make use of optical cavities to provide feedback to gain media. Conversely, mirrorless lasers can be built by using disordered structures to induce multiple scattering, which increases the effective path length in the gain medium and thus provides the necessary feedback. These so-called random lasers potentially offer a new and simple mean to address applications such as lighting. To date, they are all based on condensed-matter media. Interestingly, light or microwave amplification by stimulated emission occurs also naturally in stellar gases and planetary atmospheres. The possibility of additional scattering-induced feedback (that is, random lasing) has been discussed and could explain unusual properties of some space masers. Here, we report the experimental observation of random lasing in a controlled, cold atomic vapour, taking advantage of Raman gain. By tuning the gain frequency in the vicinity of a scattering resonance, we observe an enhancement of the light emission of the cloud due to random lasing. The unique possibility to both control the experimental parameters and to model the microscopic response of our system provides an ideal test bench for better understanding natural lasing sources, in particular the role of resonant scattering feedback in astrophysical lasers

Towards a random laser with cold atoms

Atoms can scatter light and they can also amplify it by stimulated emission. From this simple starting point, we examine the possibility of realizing a random laser in a cloud of laser-cooled atoms. The answer is not obvious as both processes (elastic scattering and stimulated emission) seem to exclude one another: pumping atoms to make them behave as amplifier reduces drastically their scattering cross-section. However, we show that even the simplest atom model allows the efficient combination of gain and scattering. Moreover, supplementary degrees of freedom that atoms offer allow the use of several gain mechanisms, depending on the pumping scheme. We thus first study these different gain mechanisms and show experimentally that they can induce (standard) lasing. We then present how the constraint of combining scattering and gain can be quantified, which leads to an evaluation of the random laser threshold. The results are promising and we draw some prospects for a practical realization of a random laser with cold atoms.Comment: Accepcted for publication by J. Opt. A for the special issue on nanolasers and random lasers (to be published early 2010

Threshold of a random laser based on Raman gain in cold atoms

We address the problem of achieving a random laser with a cloud of cold atoms, in which gain and scattering are provided by the same atoms. In this system, the elastic scattering cross-section is related to the complex atomic polarizability. As a consequence, the random laser threshold is expressed as a function of this polarizability, which can be fully determined by spectroscopic measurements. We apply this idea to experimentally evaluate the threshold of a random laser based on Raman gain between non-degenerate Zeeman states and find a critical optical thickness on the order of 200, which is within reach of state-of-the-art cold-atom experiments

Diffusion résonante de la lumière: Laser aléatoire à atomes froids et vols de Lévy des photons

In this thesis, we consider the problem of light transport in non-linear disordered media, where frequency redistribution can occur during scattering. Atomic vapors, that provide efficient resonance radiation trapping, are used as a model system to investigate the properties of two specific transport phenomena: random lasing action and Lévy flights of light. We first introduce a standard formalism to study light transport in dilute atomic vapors. The optical Bloch equations are used to obtain the density matrix of an atom subject to an external coherent field; from this matrix, we compute the atomic polarizability and cross sections used to characterize the random walk of light in the media. Most often, transport due to this random walk is well described by a diffusion equation. When gain is added to the system, the diffusive model predicts the existence of a critical size of the sample above which the diffuse intensity strongly increases. The regime above threshold is called a random laser. In the second part of this manuscript, we investigate both experimentally and theoretically the possibility to optically pump a cloud of cold atoms in order to obtain gain ; we then try to combine it with scattering to achieve random lasing action. Hyperfine Raman gain appears to be the best candidate for that purpose. In such a pumping configuration, radiation trapped inside the media has a strong impact on the sample emission properties. Most observations can be explained by radiation trapping alone, without any gain; some, however, could be attributed to a random laser. In a final part, we consider hot atomic vapors where frequency redistribution during scattering occurs in the passive system due to Doppler effect. This creates photons that are strongly detuned from the atomic transition and are weakly scattered; therefore, they can cover a large distance in the sample before being re-scattered. These rare, large steps dominate transport at a macroscopic scale and the diffusion equation no longer holds. Here, we report direct measurement of the photon jump size distribution function; we show that the obtained result is characteristic of an anomalous diffusion regime, so-called Lévy flights.Ce manuscrit s'intéresse aux phénomènes de transport de la lumière en milieux désordonnés non linéaires, dans lesquels la lumière est susceptible de subir une redistribution en fréquence lors de la diffusion. Plus spécifiquement, il étudie la possibilité d'utiliser des vapeurs atomiques éclairées à résonance pour caractériser deux régimes singuliers de transport : le laser aléatoire et les vols de Lévy des photons. Nous commençons par introduire un formalisme standard d'étude du transport de la lumière dans une vapeur atomique diluée. Les équations de Bloch optiques décrivent la réponse d'un atome à un champ extérieur cohérent; elles permettent d'obtenir les sections efficaces qui caractérisent la marche aléatoire de la lumière dans le milieu. Le plus souvent, celle-ci peut être décrite par une équation de la diffusion. En présence de gain, l'équation de la diffusion prévoit un emballement de l'intensité diffuse lorsque l'échantillon dépasse une taille critique: c'est le laser aléatoire. La seconde partie de ce manuscrit étudie par une double approche théorique et expérimentale la possibilité d'obtenir du gain dans un nuage d'atomes froids, en le soumettant à un pompage optique externe; puis de le combiner à la diffusion pour obtenir un laser aléatoire. Dans la configuration de gain Raman hyperfin, qui ressort comme la meilleure candidate à cette fin, l'impact du rayonnement diffus piégé au sein du milieu sur l'émission de l'échantillon est clairement mis en évidence. La plupart des effets observées peuvent être expliquées par un modèle de piégeage radiatif, en l'absence de gain; certaines observations, cependant, pourraient être attribuées à un laser aléatoire. Dans une dernière partie, nous nous intéressons aux vapeurs atomiques chaudes dans lesquelles une redistribution en fréquence intervient du fait de l'effet Doppler y compris en milieu passif. Le phénomène crée des photons fortement désaccordés par rapport à la résonance atomique, qui sont faiblement diffusés et peuvent parcourir de longues distances dans le milieu. Ces évènements rares de grande amplitude brisent les hypothèses du modèle diffusif. Nous présentons ici un dispositif qui permet une mesure directe de la distribution de la taille des pas des photons dans une vapeur chaude de rubidium. Le résultat obtenu est caractéristique d'un régime de diffusion anormale dit de vols de Lévy

Diffusion résonante de la lumière laser aléatoire à atomes froids et vols de Lévy des photons

Ce manuscrit s intéresse aux phénomènes de transport de la lumière en milieux désordonnées non linéaires, dans lesquels la lumière est susceptible de subir une redistribution en fréquence lors de la diffusion. Plus spécifiquement, il étudie la possibilité d utiliser des vapeurs atomiques éclairées à résonnance pour caractériser deux régions singuliers de transport : le laser aléatoire et les vols de Lévy des photons. Nous commençons par introduire un formalisme standard d étude du transport de la lumière dans une vapeur atomique diluée. Les équations de Bloch optiques décrivent la réponse d un atome à un champ extérieur cohérent ; elles permettent d obtenir les sections efficaces qui caractérisent la marche aléatoire de la lumière dans le milieu. Le plus souvent, celle-ci peut être décrite par une équation de la diffusion. En présence de gain, l équation de la diffusion prévoit un emballement de l intensité diffuse lorsque l échantillon dépasse une taille critique : c est le laser aléatoire. La seconde partie de ce manuscrit étudie par une double approche théorique et expérimentale la possibilité d obtenir du gain dans un nuage d atomes froids, en le soumettant ç un pompage optique externe ; puis de le combiner à la diffusion pour obtenir un laser aléatoire. Dans la configuration de gain Raman hyperfin, qui ressort comme la meilleure candidate à cette fin, l impact du rayonnement diffus piégé au sein du milieu sur l émission de l échantillon est clairement mis en évidence. La plupart des effets observés peuvent être expliqués par un modèle de piégeage radiatif, en l absence de gain ; certaines observations, cependant, pourraient être attribuées à un laser aléatoire. Dans une dernière partie, nous nous intéressons aux vapeurs atomiques chaudes dans lesquelles une redistribution en fréquence intervient du fait de l effet Doppler y compris en milieu passif. Le phénomène crée des photons fortement désaccordés par rapport à la résonance atomique, qui sont faiblement diffusés et peuvent parcourir de longues distances dans le milieu. Ces événements rares de grande amplitude brisent les hypothèses du modèle diffusif. Nous présentons ici un dispositif qui permet une mesure directe de la distribution de la taille des pas des photons dans une vapeur chaude de rubidium. Le résultat obtenu est caractéristique d un régime de diffusion anormal dit de vols de Lévy.In this thesis, we consider the problem of light transport in non-linear disordered media, where frequency redistribution can occur during scattering Atomic vapours, that provide efficient resonance radiation trapping, are used as a model system to investigate the properties of two specific transport phenomena : random lasing action and Lévy flights of light. We first introduce a standard formalism to study light transport in dilute atomic vapours. The optical Bloch equations are used to obtain the density matrix of an atom subject to an external coherent field ; from this matrix, we compute the atomic polarizability and cross sections used to characterize the random walk of light in the media. Most often, transport due to this random walk is well described by a diffusion equation. When gain is added to the system, the diffusive model predicts the existence of a critical size of the sample above which the diffuse intensity strongly increases. The regime above threshold is called a random laser. In the second part of this manuscript, we investigate both experimentally and theoretically the possibility to optically pump a cloud of cold atom in order to obtain gain ; we then try to combine it with scattering to achieve random lasing action. Hyperfine Raman gain appears to be the best candidate for that purpose. In such a pumping configuration, radiation trapped inside the media has a strong impact in the sample emission properties. Most observations can be explained by radiation trapping alone, without any gain ; some, however, could be attributed to a random laser. In a final part, we consider hot atomic vapors where frequency redistribution during scattering occurs in the passive systel due to Doppler effect. This creates photons that are strongly detuned from the atomic transition and are weakly scattered ; therefore, they can cover a large distance in the sample before being re-scattered. These rare, large steps dominate transport at a macroscopic scale and the diffusion equation no longer holds. Here, we report direct measurement of the photon jump size distribution function ; we show that the obtained result is characteristic of an anomalous diffusion regime, so-called Lévy flights.NICE-BU Sciences (060882101) / SudocSudocFranceF

Lévy flights of photons in hot atomic vapours

This final version is identical to the one published in Nature PhysicsInternational audienceProperties of random and fluctuating systems are often studied through the use of Gaussian distributions. However, in a number of situations, rare events have drastic consequences, which can not be explained by Gaussian statistics. Considerable efforts have thus been devoted to the study of non Gaussian fluctuations such as Lévy statistics, generalizing the standard description of random walks. Unfortunately only macroscopic signatures, obtained by averaging over many random steps, are usually observed in physical systems. We present experimental results investigating the elementary process of anomalous diffusion of photons in hot atomic vapours. We measure the step size distribution of the random walk and show that it follows a power law characteristic of Lévy flights

Detailed paragenesis and Li-mica compositions as recorders of the magmatic-hydrothermal evolution of the Maoping W-Sn deposit (Jiangxi, China)

International audienceLi-micas have been used as indicators of the evolution of granites. However, hydrothermal Li-micas are less documented. World-class W-Sn deposits associated with Early Yanshanian granites (South Jiangxi, China) show magmatic and hydrothermal Li-micas which could help unravelling the magmatic-hydrothermal evolution of rare metal deposits. Six types of Li-micas have been identified in the vein system of the Maoping W-Sn deposit through detailed petrography and EPMA and LA-ICP-MS analyses, by chronological order: (i) late-magmatic Li-micas in feldspar veins, associated with late crystallization of a peraluminous melt; (ii) hydrothermal Fe-Li micas (Fe-Li mica veins and selvages); (iii) hydrothermal Fe-Li micas in W-Sn veins; (iv) Fe-Li micas in later banded quartz veins; (v) Li-muscovite in the final stages; and finally (vi) micas associated with alteration at each stage. Based on oscillatory variations and trends in major elements composition, the chemical variations in Li-micas from the successive stages and in hydrothermal micas that crystallized in the veins are interpreted to reflect mixing between at least three fluids of possible magmatic, meteoric and metamorphic origins. The crystallization of zircons and REE minerals, combined with variations of major and trace element concentrations in the Li-micas, notably an enrichment of rare metals (W-Sn-Ta-Nb) in the Li-micas, implies emplacement of a hidden peralkaline REE-rich magma during the crystallization of the banded quartz veins, a source which was different to the pre-existing peraluminous granites. The possible involvement of both peraluminous and peralkaline intrusives suggests the existence of polyphase magmatic-hydrothermal systems in the Maoping deposit, during the Yanshanian event (190–80 Ma

Petrogenesis of Nb–(Ta) aplo-pegmatites and fine-grained granites from the Early Cretaceous Huangshan rare-metal granite suite, northeast Jiangxi Province, southeast China

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The ore-forming magmatic-hydrothermal system of the Piaotang W-Sn deposit (Jiangxi, China) as seen from Li-mica geochemistry

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