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Search Results: 1 - 10 of 14195 matches for " William Guerin "
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Mechanisms for Lasing with Cold Atoms as the Gain Medium
William Guerin,Franck Michaud,Robin Kaiser
Physics , 2008, DOI: 10.1103/PhysRevLett.101.093002
Abstract: We realize a laser with a cloud of cold rubidium atoms as gain medium, placed in a low-finesse cavity. Three different regimes of laser emission are observed corresponding respectively to Mollow, Raman and Four Wave Mixing mechanisms. We measure an output power of up to 300 $\mu$W and present the main properties of these different lasers in each regime.
Photonic properties of one-dimensionally-ordered cold atomic vapors under conditions of electromagnetically induced transparency
Alexander Schilke,Claus Zimmermann,William Guerin
Physics , 2012, DOI: 10.1103/PhysRevA.86.023809
Abstract: 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.
Subradiance in a large cloud of cold atoms
William Guerin,Michelle Araujo,Robin Kaiser
Physics , 2015,
Abstract: 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 direct 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 thickness of the sample, and is independent of the laser detuning, as expected from a coupled-dipole model.
Lévy flights of photons in hot atomic vapours
Nicolas Mercadier,William Guerin,Martine Chevrollier,Robin Kaiser
Physics , 2009, DOI: 10.1038/nphys1286
Abstract: Properties 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 L\'evy 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 L\'evy flights.
Anomalous photon diffusion in atomic vapors
Martine Chevrollier,Nicolas Mercadier,William Guerin,Robin Kaiser
Physics , 2010, DOI: 10.1140/epjd/e2010-00053-4
Abstract: 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.
Optical parametric oscillation with distributed feedback in cold atoms
Alexander Schilke,Claus Zimmermann,Philippe W. Courteille,William Guerin
Physics , 2011, DOI: 10.1038/nphoton.2011.320
Abstract: There is currently a strong interest in mirrorless lasing systems, in which the electromagnetic feedback is provided either by disorder (multiple scattering in the gain medium) or by order (multiple Bragg reflection). These mechanisms correspond, respectively, to random lasers and photonic crystal lasers. The crossover regime between order and disorder, or correlated disorder, has also been investigated with some success. Here, we report one-dimensional photonic-crystal lasing (that is, distributed feedback lasing) with a cold atom cloud that simultaneously provides both gain and feedback. The atoms are trapped in a one-dimensional lattice, producing a density modulation that creates a strong Bragg reflection with a small angle of incidence. Pumping the atoms with auxiliary beams induces four-wave mixing, which provides parametric gain. The combination of both ingredients generates a mirrorless parametric oscillation with a conical output emission, the apex angle of which is tunable with the lattice periodicity.
Microscopic characterization of Lévy flights of light in atomic vapors
Nicolas Mercadier,Martine Chevrollier,William Guerin,Robin Kaiser
Physics , 2013, DOI: 10.1103/PhysRevA.87.063837
Abstract: We investigate multiple scattering of near-resonant light in a Doppler-broadened atomic vapor. We experimentally characterize the length distribution of the steps between successive scattering events. The obtained power law is characteristic of a superdiffusive behavior, where rare but very long steps (L\'evy flights) dominate the transport properties.
Threshold of a random laser based on Raman gain in cold atoms
William Guerin,Nicolas Mercadier,Davide Brivio,Robin Kaiser
Physics , 2009, DOI: 10.1364/OE.17.011236
Abstract: 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.
A cold-atom random laser
Quentin Baudouin,Nicolas Mercadier,Vera Guarrera,William Guerin,Robin Kaiser
Physics , 2013, DOI: 10.1038/nphys2614
Abstract: 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.
Photonic Band Gaps in One-Dimensionally Ordered Cold Atomic Vapors
Alexander Schilke,Claus Zimmermann,Philippe W. Courteille,William Guerin
Physics , 2011, DOI: 10.1103/PhysRevLett.106.223903
Abstract: We experimentally investigate the Bragg reflection of light at one-dimensionally ordered atomic structures by using cold atoms trapped in a laser standing wave. By a fine tuning of the periodicity, we reach the regime of multiple reflection due to the refractive index contrast between layers, yielding an unprecedented high reflectance efficiency of 80%. This result is explained by the occurrence of a photonic band gap in such systems, in accordance with previous predictions.
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