149 research outputs found
Wireless communications with diffuse waves
Diffuse, multiple-scattered waves can be very efficient for information
transfer through disordered media, provided that antenna arrays are used for
both transmission and reception of signals. Information capacity C of a
communication channel between two identical linear arrays of n equally-spaced
antennas, placed in a disordered medium with diffuse scattering, grows linearly
with n and can attain considerable values, if antenna spacing a > lambda/2,
where lambda is the wavelength. Decrease of a below lambda/2 makes the signals
received by different antennas partially correlated, thus introducing
redundancy and reducing capacity of the communication system. When the size of
antenna arrays is well below lambda/2, the scaling of C with n becomes
logarithmic and capacity is low.Comment: RevTEX 4, 3 pages, 2 figure
Finite-size scaling analysis of localization transition for scalar waves in a 3D ensemble of resonant point scatterers
We use the random Green's matrix model to study the scaling properties of the
localization transition for scalar waves in a three-dimensional (3D) ensemble
of resonant point scatterers. We show that the probability density of
normalized decay rates of quasi-modes is very broad at the transition and
in the localized regime and that it does not obey a single-parameter scaling
law for finite system sizes that we can access. The single-parameter scaling
law holds, however, for the small- part of which we exploit to
estimate the critical exponent of the localization transition.
Finite-size scaling analysis of small- percentiles of yields an
estimate . This value is consistent with previous
results for Anderson transition in the 3D orthogonal universality class and
suggests that the localization transition under study belongs to the same
class.Comment: 11 pages, 10 figures, revised manuscrip
Spatio-temporal speckle correlations for imaging in turbid media
We discuss the far-field spatio-temporal cross-correlations of waves
multiple-scattered in a turbid medium in which is embedded a hidden
heterogeneous region (inclusion) characterized by a distinct scatterer dynamics
(as compared to the rest of the medium). We show that the spatio-temporal
correlation is affected by the inclusion which suggests a new method of imaging
in turbid media. Our results allow qualitative interpretation in terms of
diffraction theory: the cross-correlation of scattered waves behaves similarly
to the intensity of a wave diffracted by an aperture.Comment: RevTeX, 4 pages, a modified version is published in "Waves and
Imaging Through Complex Media", ed. by P. Sebbah (Kluwer Academic Publishers,
Dordrecht, 2001
Localization transition for light scattering by cold atoms in an external magnetic field
We establish a localization phase diagram for light in a random
three-dimensional (3D) ensemble of motionless two-level atoms with a three-fold
degenerate upper level, in a strong static magnetic field. Localized modes
appear in a narrow spectral band when the number density of atoms
exceeds a critical value , where is the wave
number of light in the free space. A critical exponent of the localization
transition taking place upon varying the frequency of light at a constant is estimated to be . This classifies the
transition as an Anderson localization transition of 3D orthogonal universality
class.Comment: 5 pages, 3 figures + supplemental materia
Eigenvalue distributions of large Euclidean random matrices for waves in random media
We study probability distributions of eigenvalues of Hermitian and
non-Hermitian Euclidean random matrices that are typically encountered in the
problems of wave propagation in random media.Comment: 29 pages, 10 figure
Long-range correlations of density in a Bose-Einstein condensate expanding in a random potential
We study correlations of atomic density in a weakly interacting Bose-Einstein
condensate, expanding diffusively in a random potential. We show that these
correlations are long-range and that they are strongly enhanced at long times.
Density at distant points exhibits negative correlations.Comment: 4 pages, 5 figure
Photon noise in a random laser amplifier with fluctuating properties
We study fluctuations of the number of photocounts measured by an ideal
photodetector illuminated by light scattered in an amplifying disordered
medium, below the threshold for random lasing. We show that the variance of
fluctuations and their correlation function carry information about fluctuating
properties of the medium. A direct link is established between the fluctuations
of the number of photocounts due to the amplified spontaneous emission (ASE)
and the dimensionless conductance g of the medium. Our results suggest a
possibility of probing amplifying disordered media by analyzing statistics of
their ASE, without illuminating them from outside by a probe beam.Comment: 14 pages, 9 figure
Ioffe-Regel criterion of Anderson localization in the model of resonant point scatterers
We establish a phase diagram of a model in which scalar waves are scattered
by resonant point scatterers pinned at random positions in the free
three-dimensional (3D) space. A transition to Anderson localization takes place
in a narrow frequency band near the resonance frequency provided that the
number density of scatterers exceeds a critical value , where is the wave number in the free space. The
localization condition can be rewritten as ,
where is the on-resonance mean free path in the independent-scattering
approximation. At mobility edges, the decay of the average amplitude of a
monochromatic plane wave is not purely exponential and the growth of its phase
is nonlinear with the propagation distance. This makes it impossible to define
the mean free path and the effective wave number in a usual way. If
the latter are defined as an effective decay length of the intensity and an
effective growth rate of the phase of the average wave field, the Ioffe-Regel
parameter at the mobility edges can be calculated and takes values
from 0.3 to 1.2 depending on . Thus, the Ioffe-Regel criterion of
localization is valid only
qualitatively and cannot be used as a quantitative condition of Anderson
localization in 3D.Comment: Revised and extended version. 9 pages, 6 figure
Finite-time scaling at the Anderson transition for vibrations in solids
A model in which a three-dimensional elastic medium is represented by a
network of identical masses connected by springs of random strengths and
allowed to vibrate only along a selected axis of the reference frame, exhibits
an Anderson localization transition. To study this transition, we assume that
the dynamical matrix of the network is given by a product of a sparse random
matrix with real, independent, Gaussian-distributed non-zero entries and its
transpose. A finite-time scaling analysis of system's response to an initial
excitation allows us to estimate the critical parameters of the localization
transition. The critical exponent is found to be in
agreement with previous studies of Anderson transition belonging to the
three-dimensional orthogonal universality class.Comment: Revised manuscript. 8 pages, 5 figure
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