2,028 research outputs found
Source localization in random acoustic waveguides
Mode coupling due to scattering by weak random inhomogeneities in waveguides leads to loss of coherence of wave fields at long distances of propagation. This in turn leads to serious deterioration of coherent source localization methods, such as matched field. We study with analysis and numerical simulations how such deterioration occurs and introduce a novel incoherent approach for long range source localization in random waveguides. It is based on a special form of transport theory for the incoherent fluctuations of the wave field. We study theoretically the statistical stability of the method and illustrate its performance with numerical simulations. We also show how it can be used to estimate the correlation function of the random fluctuations of the wave speed
Statistics and control of waves in disordered media
Fundamental concepts in the quasi-one-dimensional geometry of disordered
wires and random waveguides in which ideas of scaling and the transmission
matrix were first introduced are reviewed. We discuss the use of the
transmission matrix to describe the scaling, fluctuations, delay time, density
of states, and control of waves propagating through and within disordered
systems. Microwave measurements, random matrix theory calculations, and
computer simulations are employed to study the statistics of transmission and
focusing in single samples and the scaling of the probability distribution of
transmission and transmittance in random ensembles. Finally, we explore the
disposition of the energy density of transmission eigenchannels inside random
media.Comment: 28 Pages, 18 Figures (Review
Carrier drift velocity and edge magnetoplasmons in graphene
We investigate electron dynamics at the graphene edge by studying the
propagation of collective edge magnetoplasmon (EMP) excitations. By timing the
travel of narrow wave-packets on picosecond time scales around exfoliated
samples, we find chiral propagation with low attenuation at a velocity which is
quantized on Hall plateaus. We extract the carrier drift contribution from the
EMP propagation and find it to be slightly less than the Fermi velocity, as
expected for an abrupt edge. We also extract the characteristic length for
Coulomb interaction at the edge and find it to be smaller than for soft,
depletion edge systems.Comment: 5 pages, 3 figures of main text and 6 pages, 6 figures of
supplemental materia
Dynamics of localization in a waveguide
This is a review of the dynamics of wave propagation through a disordered
N-mode waveguide in the localized regime. The basic quantities considered are
the Wigner-Smith and single-mode delay times, plus the time-dependent power
spectrum of a reflected pulse. The long-time dynamics is dominated by resonant
transmission over length scales much larger than the localization length. The
corresponding distribution of the Wigner-Smith delay times is the Laguerre
ensemble of random-matrix theory. In the power spectrum the resonances show up
as a 1/t^2 tail after N^2 scattering times. In the distribution of single-mode
delay times the resonances introduce a dynamic coherent backscattering effect,
that provides a way to distinguish localization from absorption.Comment: 18 pages including 8 figures; minor correction
Loss of Resolution for the Time Reversal of Waves in Random Underwater Acoustic Channels
In this paper we analyze a time-reversal experiment in a random underwater
acoustic channel. In this kind of waveguide with semi-infinite cross section a
propagating field can be decomposed over three kinds of modes: the propagating
modes, the radiating modes and the evanescent modes. Using an asymptotic
analysis based on a separation of scales technique we derive the asymptotic
form of the the coupled mode power equation for the propagating modes. This
approximation is used to compute the transverse profile of the refocused field
and show that random inhomogeneities inside the waveguide deteriorate the
spatial refocusing. This result, in an underwater acoustic channel context, is
in contradiction with the classical results about time-reversal experiment in
other configurations, for which randomness in the propagation medium enhances
the refocusing.Comment: 31 pages, 11 figure
- …