1,738 research outputs found
A Bose-Einstein condensate in a random potential
An optical speckle potential is used to investigate the static and dynamic
properties of a Bose-Einstein condensate in the presence of disorder. For
strong disorder the condensate is localized in the deep wells of the potential.
With smaller levels of disorder, stripes are observed in the expanded density
profile and strong damping of dipole and quadrupole oscillations is seen.
Uncorrelated frequency shifts of the two modes are measured for a weak disorder
and are explained using a sum-rules approach and by the numerical solution of
the Gross-Pitaevskii equation
Effect of optical disorder and single defects on the expansion of a Bose-Einstein condensate in a one-dimensional waveguide
We investigate the one-dimensional expansion of a Bose-Einstein condensate in
an optical guide in the presence of a random potential created with optical
speckles. With the speckle the expansion of the condensate is strongly
inhibited. A detailed investigation has been carried out varying the
experimental conditions and checking the expansion when a single optical defect
is present. The experimental results are in good agreement with numerical
calculations based on the Gross-Pitaevskii equation.Comment: 5 pages, 5 figure
Theory for the photon statistics of random lasers
A theory for the photon statistics of a random laser is presented. Noise is
described by Langevin operators, where both fluctuations of the electromagnetic
field and of the medium are included. The theory is valid for all lasers with
small outcoupling when the laser cavity is large compared to the wavelength of
the radiation. The theory is applied to a chaotic laser cavity with a small
opening. It is known that a large number of modes can be above threshold
simultaneously in such a cavity. It is shown the amount of fluctuations is
increased compared to the Poissonian value by an amount that depends on that
number
Distributed leadership, trust and online communities
This paper analyses the role of distributed leadership and trust in online communities. The team-based informal ethos of online collaboration requires a different kind of leadership from that in formal positional hierarchies. Such leadership may be more flexible and sophisticated, capable of encompassing ambiguity and rapid change. Online leaders need to be partially invisible, delegating power and distributing tasks. Yet, simultaneously, online communities are facilitated by the high visibility and subtle control of expert leaders. This paradox: that leaders need to be both highly visible and invisible as appropriate, was derived from prior research and tested in the analysis of online community discussions using a pattern-matching process. It is argued that both leader visibility and invisibility are important for the facilitation of trusting collaboration via distributed leadership. Advanced leadership responses to complex situations in online communities foster positive group interaction and decision-making, facilitated through active distribution of specific tasks
Time Dependent Theory for Random Lasers
A model to simulate the phenomenon of random lasing is presented. It couples
Maxwell's equations with the rate equations of electronic population in a
disordered system. Finite difference time domain methods are used to obtain the
field pattern and the spectra of localized lasing modes inside the system. A
critical pumping rate exists for the appearance of the lasing
peaks. The number of lasing modes increase with the pumping rate and the length
of the system. There is a lasing mode repulsion. This property leads to a
saturation of the number of modes for a given size system and a relation
between the localization length and average mode length .Comment: 8 pages. Send to PR
Decay Rate Distributions of Disordered Slabs and Application to Random Lasers
We compute the distribution of the decay rates (also referred to as residues)
of the eigenstates of a disordered slab from a numerical model. From the
results of the numerical simulations, we are able to find simple analytical
formulae that describe those results well. This is possible for samples both in
the diffusive and in the localised regime. As example of a possible
application, we investigate the lasing threshold of random lasers.Comment: 11 pages, 11 figure
Smoothing effect and delocalization of interacting Bose-Einstein condensates in random potentials
We theoretically investigate the physics of interacting Bose-Einstein
condensates at equilibrium in a weak (possibly random) potential. We develop a
perturbation approach to derive the condensate wavefunction for an amplitude of
the potential smaller than the chemical potential of the condensate and for an
arbitrary spatial variation scale of the potential. Applying this theory to
disordered potentials, we find in particular that, if the healing length is
smaller than the correlation length of the disorder, the condensate assumes a
delocalized Thomas-Fermi profile. In the opposite situation where the
correlation length is smaller than the healing length, we show that the random
potential can be significantly smoothed and, in the meanfield regime, the
condensate wavefunction can remain delocalized, even for very small correlation
lengths of the disorder.Comment: The word "screening" has been changed to "smoothing" to avoid
confusions with other effects discussed in the literature. This does not
affect the content of paper, nor the results, nor the physical discussio
Determination of the diffusion constant using phase-sensitive measurements
We apply a pulsed-light interferometer to measure both the intensity and the
phase of light that is transmitted through a strongly scattering disordered
material. From a single set of measurements we obtain the time-resolved
intensity, frequency correlations and statistical phase information
simultaneously. We compare several independent techniques of measuring the
diffusion constant for diffuse propagation of light. By comparing these
independent measurements, we obtain experimental proof of the consistency of
the diffusion model and corroborate phase statistics theory.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
Time-resolved broadband analysis of slow-light propagation and superluminal transmission of electromagnetic waves in three-dimensional photonic crystals
A time-resolved analysis of the amplitude and phase of THz pulses propagating
through three-dimensional photonic crystals is presented. Single-cycle pulses
of THz radiation allow measurements over a wide frequency range, spanning more
than an octave below, at and above the bandgap of strongly dispersive photonic
crystals. Transmission data provide evidence for slow group velocities at the
photonic band edges and for superluminal transmission at frequencies in the
gap. Our experimental results are in good agreement with
finite-difference-time-domain simulations.Comment: 7 pages, 11 figure
Super-reflection of light from a random amplifying medium with disorder in the complex refractive index : Statistics of fluctuations
The probability distribution of the reflection coefficient for light
reflected from a one-dimensional random amplifying medium with {\it
cross-correlated} spatial disorder in the real and the imaginary parts of the
refractive index is derived using the method of invariant imbedding. The
statistics of fluctuations have been obtained for both the correlated telegraph
noise and the Gaussian white-noise models for the disorder. In both cases, an
enhanced backscattering (super-reflection with reflection coefficient greater
than unity) results because of coherent feedback due to Anderson localization
and coherent amplification in the medium. The results show that the effect of
randomness in the imaginary part of the refractive index on localization and
super-reflection is qualitatively different.Comment: RevTex 6 pages, 3 figures in ps file
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