Decay dynamics in the coupled-dipole model

Cooperative scattering in cold atoms has gained renewed interest, in particular in the context of single-photon superradiance, with the recent experimental observation of super-and subradiance in dilute atomic clouds. Numerical simulations to support experimental signatures of cooperative scattering are often limited by the number of dipoles which can be treated, well below the number of atoms in the experiments. In this paper, we provide systematic numerical studies aimed at matching the regime of dilute atomic clouds. We use a scalar coupled-dipole model in the low excitation limit and an exclusion volume to avoid density-related effects. Scaling laws for super-and subradiance are obtained and the limits of numerical studies are pointed out. We also illustrate the cooperative nature of light scattering by considering an incident laser field, where half of the beam has a $\pi$ phase shift. The enhanced subradiance obtained under such condition provides an additional signature of the role of coherence in the detected signal

Subradiance in a Large Cloud of Cold Atoms

Since Dicke's seminal paper on coherence in spontaneous radiation by atomic ensembles, superradiance has been extensively studied. Subradiance, on the contrary, has remained elusive, mainly because subradiant states are weakly coupled to the environment and are very sensitive to nonradiative decoherence processes.Here we report the experimental observation of subradiance in an extended and dilute cold-atom sample containing a large number of particles. We use a far detuned laser to avoid multiple scattering and observe the temporal decay after a sudden switch-off of the laser beam. After the fast decay of most of the fluorescence, we detect a very slow decay, with time constants as long as 100 times the natural lifetime of the excited state of individual atoms. This subradiant time constant scales linearly with the cooperativity parameter, corresponding to the on-resonance optical depth of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model

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

Superradiance in a Large and Dilute Cloud of Cold Atoms in the Linear-Optics Regime

Superradiance has been extensively studied in the 1970s and 1980s in the regime of superfluores-cence, where a large number of atoms are initially excited. Cooperative scattering in the linear-optics regime, or "single-photon superradiance" , has been investigated much more recently, and superra-diant decay has also been predicted, even for a spherical sample of large extent and low density, where the distance between atoms is much larger than the wavelength. Here, we demonstrate this effect experimentally by directly measuring the decay rate of the off-axis fluorescence of a large and dilute cloud of cold rubidium atoms after the sudden switch-off of a low-intensity laser driving the atomic transition. We show that, at large detuning, the decay rate increases with the on-resonance optical depth. In contrast to forward scattering, the superradiant decay of off-axis fluorescence is suppressed near resonance due to attenuation and multiple-scattering effects

PHYSIOLOGY AND ESTUARINE ECOLOGY OF PHENANTHRENE-DEGRADING BACTERIA (MYCOBACTERIUM, MICROLAYER, METABOLITES, PAH, MARINAS, NEW HAMPSHIRE)

Using radiorespirometric, spectrophotometric and high performance liquid and thin layer chromatographic methods, the degradation and intermediary metabolism of the polycyclic aromatic hydrocarbon (PAH), phenanthrene, by estuarine enrichment and pure microbial cultures was examined. A Mycobacterium species, strain BG1, able to use phenanthrene as sole carbon and energy source, was isolated from estuarine sediment. Phenanthrene degradation proceeded via the intermediates, 1-hydroxy-2-naphthoic acid (1H2NA) and protocatechuic acid. However, unlike other phenanthrene-degrading cultures, aromatic intermediates, including 1H2NA, did not accumulate. Consistent with the induction of meta pathway enzymes in phenanthrene-grown BG1 cells, phenanthrene degradation was stimulated in pyruvate-supplemented cultures (an end product of meta cleavage) and repressed in succinate-supplemented cultures (an end product of ortho cleavage). Phenanthrene-degrading cells possessed 3 plasmids (20.6, 57.5 and 76.7 megadaltons) likely responsible for degradation. Plasmids and the phenanthrene-degrading phenotype were absent in nutrient-grown cells. Analogous to degradative reactions involving other PAH when present in excess, enrichment cultures and cultures of most isolates derived from them accumulated near stoichiometric amounts of 1 H2NA during phenanthrene degradation. In pure cultures, 1 H2NA was not further degraded. In enrichment cultures, subsequent mineralization of 1 H2NA led to secondary increases in biomass. Two-stage mineralization of phenanthrene was also evidenced by a biphasic (\u2714)CO(,2) production in enrichment cultures spiked with low concentrations (0.5 mg L(\u27-1)) of (\u2714)C-phenanthrene. Here, however, polar metabolites comprised less than 10% of the total initial activity. Phenanthrene-degrading bacteria were ubiquitous in the waters and sediments of the Great Bay Estuary, NH, and activities correlated positively with the degree of previous exposure to PAH. Particularly active were sediments collected near an oil refinery and water samples collected downstream from a dredging operation. Coal tar-derived PAH, analyzed in dredge and downstream sediments by capillary gas chromatography, were introduced into the river in high concentrations over several months. Phenanthrene degradation potentials were also high in areas influenced by pleasure and commercial boating activities. Surface microlayer samples from marinas were enriched with fluorescent hydrocarbons but often showed depressed phenanthrene degradative activities relative to underlying bulk waters. Sunlight, hydrocarbons or organotin compounds were possibly inhibitory

Photonic properties of one-dimensionally-ordered cold atomic vapors under conditions of electromagnetically induced transparency

We experimentally study the photonic properties of a cold-atom sample trapped in a one-dimensional optical lattice under the conditions of electromagnetically induced transparency. We show that such a medium has two photonic band gaps. One of them is in the transparency window and gives rise to a Bragg mirror, which is spectrally very narrow and dynamically tunable. We discuss the advantages and the limitations of this system. As an illustration of a possible application we demonstrate a two-port all-optical switch

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

Comparison of three approaches to light scattering by dilute cold atomic ensembles

Collective effects in atom-light interaction is of great importance for cold-atom-based quantum devices or fundamental studies on light transport in complex media. Here we discuss and compare three different approaches to light scattering by dilute cold atomic ensembles. The first approach is a coupled-dipole model, valid at low intensity, which includes cooperative effects, like superradiance, and other coherent properties. The second one is a random-walk model, which includes classical multiple scattering and neglects coherence effects. The third approach is a crude approximation only based on the attenuation of the excitation beam inside the medium, the so-called "shadow effect". We show that in the case of a low-density sample, the random walk approach is an excellent approximation for steady-state light scattering, and that the shadow effect surprisingly gives rather accurate results at least up to optical depths on the order of 15

Noise spectroscopy with large clouds of cold atoms

Noise measurement is a powerful tool to investigate many phenomena from laser characterization to quantum behavior of light. In this paper, we report on intensity noise measurements obtained when a laser beam is transmitted through a large cloud of cold atoms. While this measurement could possibly investigate complex processes such as the influence of atomic motion, one is first limited by the conversion of the intrinsic laser frequency noise to intensity noise via the atomic resonance. This conversion is studied here in details. We show that, while experimental intensity noise spectra collapse onto the same curve at low Fourier frequencies, some differences appear at higher frequencies when the probe beam is detuned from the center of the resonance line. A simple model, based on a mean-field approach, which corresponds to describing the atomic cloud by a dielectric susceptibility, is sufficient to understand the main features. Using this model, the noise spectra allow extracting some quantitative informations on the laser noise as well as on the atomic sample