753 research outputs found
Exploiting pattern transformation to tune phononic band gaps in a two-dimensional granular crystal
The band structure of a two-dimensional granular crystal composed of silicone rubber and polytetrafluoroethylene (PTFE) cylinders is investigated numerically. This system was previously shown to undergo a pattern transformation with uniaxial compression by Göncü et al. [Soft Matter 7, 2321 (2011)]. The dispersion relations of the crystal are computed at different levels of deformation to demonstrate the tunability of the band structure, which is strongly affected by the pattern transformation that induces new band gaps. Replacement of PTFE particles with rubber ones reveals that the change of the band structure is essentially governed by pattern transformation rather than particles¿ mechanical properties
Sonic crystal lenses that obey Lensmaker's formula
This paper presents a theoretical study of the phenomenon of acoustic imaging
by sonic crystals, which are made of two-dimensional regular arrays of rigid
cylinders placed in parallel in air. The scattering of acoustic waves is
computed using the standard multiple scattering theory, and the band structures
are computed by the plane-wave expansion method. It is shown that properly
arranged arrays not only can behave as acoustic lenses, but also the focusing
effect can be well described by Lensmaker's formula. Possible applications are
also discussed.Comment: 4 pages, 5 figure
Resonant acousto-optics in the terahertz range: TO-phonon polaritons driven by an ultrasonic wave
The resonant acousto-optic effect is studied both analytically and
numerically in the terahertz range where the transverse-optical (TO) phonons
play the role of a mediator which strongly couples the ultrasound and light
fields. A propagating acoustic wave interacts with the TO phonons via
anharmonic channels and opens band gaps in the TO-phonon polariton energy
dispersion that results in pronounced Bragg scattering and reflection of the
incoming light. The separation in frequency of different Bragg replicas, which
is at the heart of acousto-optics, allows us to study the resonant
acousto-optic effect in the most simple and efficient geometry of collinear
propagation of electromagnetic and ultrasonic waves. The acoustically induced
energy gaps, Bragg reflection spectra, and the spatial distribution of the
electric field and polarization are calculated for CuCl parameters, in a wide
range of frequencies and intensities of the pumping acoustic wave. Our results
show drastic changes in terahertz spectra of semiconductor crystals that opens
the way for efficient and accessible manipulation of their infrared properties,
by tuning the parameters of the acoustic wave.Comment: 20 pages, 14 figure
Approximate Analytic Solution for the Spatiotemporal Evolution of Wave Packets undergoing Arbitrary Dispersion
We apply expansion methods to obtain an approximate expression in terms of
elementary functions for the space and time dependence of wave packets in a
dispersive medium. The specific application to pulses in a cold plasma is
considered in detail, and the explicit analytic formula that results is
provided. When certain general initial conditions are satisfied, these
expressions describe the packet evolution quite well. We conclude by employing
the method to exhibit aspects of dispersive pulse propagation in a cold plasma,
and suggest how predicted and experimental effects may be compared to improve
the theoretical description of a medium's dispersive properties.Comment: 17 pages, 4 figures, RevTe
Scattering of a Dirac electron on a mass barrier
The interaction of a wave packet (and in particular the wave front) with a
mass barrier is investigated in one dimension. We discuss the main features of
the wave packet that are inherent to two-dimensional wave packets, such as
compression during reflection, penetration in the case when the energy is lower
than the height of the barrier, waving tails, precursors, and the retardation
of the reflected and penetrated wave packets. These features depend on the
wave-packet envelope function which we demonstrate by considering the case of a
rectangular wave packet with sharp front and trailing edges and a smooth
Gaussian wave packet. The method of Fourier integral for obtaining the
nonstationary solutions is used.Comment: 12 pages, 9 figure
Information hiding and retrieval in Rydberg wave packets using half-cycle pulses
We demonstrate an information hiding and retrieval scheme with the relative
phases between states in a Rydberg wave packet acting as the bits of a data
register. We use a terahertz half-cycle pulse (HCP) to transfer phase-encoded
information from an optically accessible angular momentum manifold to another
manifold which is not directly accessed by our laser pulses, effectively hiding
the information from our optical interferometric measurement techniques. A
subsequent HCP acting on these wave packets reintroduces the information back
into the optically accessible data register manifold which can then be `read'
out.Comment: 4 pages, 4 figure
Continuous measurements in a composite quantum system and possible exchange of information between its parts
We study an influence of the continuous measurement in a composite quantum
system C on the evolution of the states of its parts. It is shown that the
character of the evolution (decoherence or recoherence) depends on the type of
the measured quantity and on the initial state of the system. A number of
conditions under which the states of the subsystems of C decohere during the
measuring process are established. We propose a model of the composite system
and specify the observable the measurement of which may result in the
recoherence of the state of one of the subsystems of C. In the framework of
this model we find the optimal regime for the exchange of information between
the parts of C during the measurement. The main characteristics of such a
process are computed. We propose a scheme of detection of the recoherence under
the measurement in a concrete physical experiment.Comment: 6 page
Inhomogeneous DNA: conducting exons and insulating introns
Parts of DNA sequences known as exons and introns play very different role in
coding and storage of genetic information. Here we show that their conducting
properties are also very different. Taking into account long-range correlations
among four basic nucleotides that form double-stranded DNA sequence, we
calculate electron localization length for exon and intron regions. Analyzing
different DNA molecules, we obtain that the exons have narrow bands of extended
states, unlike the introns where all the states are well localized. The band of
extended states is due to a specific form of the binary correlation function of
the sequence of basic DNA nucleotides.Comment: 14 pages, 6 figure
Recovering the stationary phase condition for accurately obtaining scattering and tunneling times
The stationary phase method is often employed for computing tunneling {\em
phase} times of analytically-continuous {\em gaussian} or infinite-bandwidth
step pulses which collide with a potential barrier. The indiscriminate
utilization of this method without considering the barrier boundary effects
leads to some misconceptions in the interpretation of the phase times. After
reexamining the above barrier diffusion problem where we notice the wave packet
collision necessarily leads to the possibility of multiple reflected and
transmitted wave packets, we study the phase times for tunneling/reflecting
particles in a framework where an idea of multiple wave packet decomposition is
recovered. To partially overcome the analytical incongruities which rise up
when tunneling phase time expressions are obtained, we present a theoretical
exercise involving a symmetrical collision between two identical wave packets
and a one dimensional squared potential barrier where the scattered wave
packets can be recomposed by summing the amplitudes of simultaneously reflected
and transmitted waves.Comment: 32 pages, 5 figures, 1 tabl
On the electromagnetic properties of active media
Several results concerning active media or metamaterials are proved and
discussed. In particular, we consider the permittivity, permeability, wave
vector, and refractive index, and discuss stability, refraction, gain, and
fundamental limitations resulting from causality
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