87 research outputs found
Far field subwavelength imaging and focusing using a wire medium based resonant metalens
This is the second article in a series of two dealing with the concept of
"resonant metalens" we introduced recently [Phys. Rev. Lett. 104, 203901
(2010)]. It is a new type of lens capable of coding in time and radiating
efficiently in the far field region sub-diffraction information of an object. A
proof of concept of such a lens is performed in the microwave range, using a
medium made out of a square lattice of parallel conducting wires with finite
length. We investigate a sub-wavelength focusing scheme with time reversal and
demonstrate experimentally spots with focal widths of {\lambda}/25. Through a
cross-correlation based imaging procedure we show an image reconstruction with
a resolution of {\lambda}/80. Eventually we discuss the limitations of such a
lens which reside essentially in losses
Information transfer through disordered media by diffuse waves
We consider the information content h of a scalar multiple-scattered, diffuse
wave field and the information capacity C of a communication
channel that employs diffuse waves to transfer the information through a
disordered medium. Both h and C are shown to be directly related to the
mesoscopic correlations between the values of at different
positions in space, arising due to the coherent nature of the wave.
For the particular case of a communication channel between two identical linear
arrays of equally-spaced transmitters/receivers (receiver spacing a),
we show that the average capacity and obtain explicit analytic
expressions for in the limit of and ,
where , is the wavelength, and is the mean
free path. Modification of the above results in the case of finite but large n
and is discussed as well.Comment: REVTeX 4, 12 pages, 7 figure
Recurrent scattering and memory effect at the Anderson localization transition
We report on ultrasonic measurements of the propagation operator in a
strongly scattering mesoglass. The backscattered field is shown to display a
deterministic spatial coherence due to a remarkably large memory effect induced
by long recurrent trajectories. Investigation of the recurrent scattering
contribution directly yields the probability for a wave to come back close to
its starting spot. The decay of this quantity with time is shown to change
dramatically near the Anderson localization transition. The singular value
decomposition of the propagation operator reveals the dominance of very intense
recurrent scattering paths near the mobility edge.Comment: 5 pages, 4 figure
Exploiting disorder for perfect focusing
We demonstrate experimentally that disordered scattering can be used to
improve, rather than deteriorate, the focusing resolution of a lens. By using
wavefront shaping to compensate for scattering, light was focused to a spot as
small as one tenth of the diffraction limit of the lens. We show both
experimentally and theoretically that it is the scattering medium, rather than
the lens, that determines the width of the focus. Despite the disordered
propagation of the light, the profile of the focus was always exactly equal to
the theoretical best focus that we derived.Comment: 4 pages, 4 figure
Time-domain numerical simulations of multiple scattering to extract elastic effective wavenumbers
Elastic wave propagation is studied in a heterogeneous 2-D medium consisting
of an elastic matrix containing randomly distributed circular elastic
inclusions. The aim of this study is to determine the effective wavenumbers
when the incident wavelength is similar to the radius of the inclusions. A
purely numerical methodology is presented, with which the limitations usually
associated with low scatterer concentrations can be avoided. The elastodynamic
equations are integrated by a fourth-order time-domain numerical scheme. An
immersed interface method is used to accurately discretize the interfaces on a
Cartesian grid. The effective field is extracted from the simulated data, and
signal-processing tools are used to obtain the complex effective wavenumbers.
The numerical reference solution thus-obtained can be used to check the
validity of multiple scattering analytical models. The method is applied to the
case of concrete. A parametric study is performed on longitudinal and
transverse incident plane waves at various scatterers concentrations. The phase
velocities and attenuations determined numerically are compared with
predictions obtained with multiple scattering models, such as the Independent
Scattering Approximation model, the Waterman-Truell model, and the more recent
Conoir-Norris model.Comment: Waves in Random and Complex Media (2012) XX
Shaping speckles: spatio-temporal focussing of an ultrafast pulse through a multiply scattering medium
The multiple scattering of coherent light is a problem of both fundamental
and applied importance. In optics, phase conjugation allows spatial focussing
and imaging through a multiply scattering medium; however, temporal control is
nonetheless elusive, and multiple scattering remains a challenge for
femtosecond science. Here, we report on the spatially and temporally resolved
measurement of a speckle field produced by the propagation of an ultrafast
optical pulse through a thick strongly scattering medium. Using spectral pulse
shaping, we demonstrate the spatially localized temporal recompression of the
output speckle to the Fourier-limit duration, offering an optical analogue to
time-reversal experiments in the acoustic regime. This approach shows that a
multiply scattering medium can be put to profit for light manipulation at the
femtosecond scale, and has a diverse range of potential applications that
includes quantum control, biological imaging and photonics.Comment: 7 pages, 3 figures, published in Nature Communication
Decoherence as Decay of the Loschmidt Echo in a Lorentz Gas
Classical chaotic dynamics is characterized by the exponential sensitivity to
initial conditions. Quantum mechanics, however, does not show this feature. We
consider instead the sensitivity of quantum evolution to perturbations in the
Hamiltonian. This is observed as an atenuation of the Loschmidt Echo, ,
i.e. the amount of the original state (wave packet of width ) which is
recovered after a time reversed evolution, in presence of a classically weak
perturbation. By considering a Lorentz gas of size , which for large is
a model for an {\it unbounded} classically chaotic system, we find numerical
evidence that, if the perturbation is within a certain range, decays
exponentially with a rate determined by the Lyapunov exponent
of the corresponding classical dynamics. This exponential decay
extends much beyond the Eherenfest time and saturates at a time
, where is the effective dimensionality of the Hilbert space. Since quantifies the increasing uncontrollability of the quantum phase
(decoherence) its characterization and control has fundamental interest.Comment: 3 ps figures, uses Revtex and epsfig. Major revision to the text, now
including discussion and references on averaging and Ehrenfest time. Figures
2 and 3 content and order change
Focusing and Compression of Ultrashort Pulses through Scattering Media
Light scattering in inhomogeneous media induces wavefront distortions which
pose an inherent limitation in many optical applications. Examples range from
microscopy and nanosurgery to astronomy. In recent years, ongoing efforts have
made the correction of spatial distortions possible by wavefront shaping
techniques. However, when ultrashort pulses are employed scattering induces
temporal distortions which hinder their use in nonlinear processes such as in
multiphoton microscopy and quantum control experiments. Here we show that
correction of both spatial and temporal distortions can be attained by
manipulating only the spatial degrees of freedom of the incident wavefront.
Moreover, by optimizing a nonlinear signal the refocused pulse can be shorter
than the input pulse. We demonstrate focusing of 100fs pulses through a 1mm
thick brain tissue, and 1000-fold enhancement of a localized two-photon
fluorescence signal. Our results open up new possibilities for optical
manipulation and nonlinear imaging in scattering media
Universality of the Lyapunov regime for the Loschmidt echo
The Loschmidt echo (LE) is a magnitude that measures the sensitivity of
quantum dynamics to perturbations in the Hamiltonian. For a certain regime of
the parameters, the LE decays exponentially with a rate given by the Lyapunov
exponent of the underlying classically chaotic system. We develop a
semiclassical theory, supported by numerical results in a Lorentz gas model,
which allows us to establish and characterize the universality of this Lyapunov
regime. In particular, the universality is evidenced by the semiclassical limit
of the Fermi wavelength going to zero, the behavior for times longer than
Ehrenfest time, the insensitivity with respect to the form of the perturbation
and the behavior of individual (non-averaged) initial conditions. Finally, by
elaborating a semiclassical approximation to the Wigner function, we are able
to distinguish between classical and quantum origin for the different terms of
the LE. This approach renders an understanding for the persistence of the
Lyapunov regime after the Ehrenfest time, as well as a reinterpretation of our
results in terms of the quantum--classical transition.Comment: 33 pages, 17 figures, uses Revtex
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