177,416 research outputs found
Generation of arbitrary radially polarized array beams by modulating the correlation structure
We demonstrate a convenient approach for simultaneously manipulating the
amplitude and polarization of light beams by means of the modulation of the
correlation structure. As an illustration, we constructed a periodic
correlation structure that can generate an arbitrary radially polarized array
(RPA) beam of a radial or rectangular symmetry array in the focal plane from a
radially polarized (RP) beam. The physical realizability conditions for such
source and the far-field beam condition are derived. It is illustrated that the
beamlet shape and the state of polarization (SOP) can be effectively controlled
by the initial correlation structure and the coherence width. Furthermore, by
designing the source correlation structure, a tunable OK-shaped RPA beam and an
optical cage are demonstrated, which can find widespread applications in
non-destructive manipulation of particles and living biological cells. The
arbitrariness in the design of correlation structure prompted us to find more
convenient approaches for controlling the statistics of light beams in terms of
amplitude and polarization
The reversibility of the Goos-H\"anchen shift near the band-crossing structure of one-dimemsional photonic crystals containing left-handed metamaterials
We perform a theoretical investigation on the Goos-H\"achen (GH) shift in
one-dimensional photonic crystals (1DPCs) containing left-handed metamaterials
(LHMs). We find an unusal effect of the GH shift near the photonic
band-crossing structure, which is located at the condition, , under the inclined
incident angle, here A denotes the LHM layer and B denotes the dielectric
layer. Above the frequency of the band-crossing point (BCP), the GH shift
changes from negative to positive as the incident angle increases, while the GH
shift changes reversely below the BCP frequency. This effect is explained in
terms of the phase property of the band-crossing structure.Comment: 4 pages, 5 figure
Electronic Band gaps and transport properties inside graphene superlattices with one-dimensional periodic squared potentials
The electronic transport properties and band structures for the
graphene-based one-dimensional (1D) superlattices with periodic squared
potentials are investigated. It is found that a new Dirac point is formed,
which is exactly located at the energy which corresponds to the zero (volume)
averaged wavenumber inside the 1D periodic potentials. The location of such a
new Dirac point is robust against variations in the lattice constants, and it
is only dependent on the ratio of potential widths. The zero-averaged
wavenumber gap associated with the new Dirac point is insensitive to both the
lattice constant and the structural disorder, and the defect mode in the
zero-averaged wavenumber gap is weakly dependent on the insident angles of
carriers.Comment: 7 figure
General decay for a viscoelastic wave equation with dynamic boundary conditions and a time-varying delay
The goal of this paper is to study a nonlinear viscoelastic wave equation
with strong damping, time-varying delay and dynamical boundary condition. By
introducing suitable energy and Lyapunov functionals, under suitable
assumptions, we then prove a general decay result of the energy, from which the
usual exponential and polynomial decay rates are only special cases
Fidelity, purity and entanglement of two-mode spatially Gaussian-entangled light fields in Turbulence Atmosphere
In this paper, we investigate the propagation of two-mode spatially
Gaussian-entangled quantum light fields passing through the turbulence
atmosphere. From the propagation formula of the two-mode wave function in the
position representation, we have derived the analytical expressions for the
fidelity, purity and logarithmic negativity (entanglement) of the resulting
quantum state after the long-distance atmospheric transportation. Based on the
derived formulae, the effects of the atmospheric turbulences on the evolutions
of quantum properties of the resulting two-mode quantum state are discussed in
detail under different input parameters of the initial two-mode quantum state.
The results show that the maximal distributing distance of quantum entanglement
is strongly dependent on the atmospheric conditions: when the atmospheric
turbulence becomes stronger and stronger, the maximal distance becomes shorter
and shorter, and both the fidelity and purity decrease quicker and quicker as
functions of propagating distances. Under a certain atmospheric condition, with
the increasing of the input entanglement of the initial two-mode spatially
Gaussian-entangled quantum state, the maximal distributing distance for
preserving the entanglement gradually increases and always has a saturated
(upper) limitation, and both the evolutions of the fidelity and purity are
affected by the input parameters of the initial two-mode quantum state, Finally
the optimal parameters of the input two-mode quantum state with the fixed input
entanglement are discussed in order to obtain the optimal transfer distribution
of the quantum entanglement over a long distance under a certain atmosphere.
Our theoretical results are very helpful for building the distribution of the
quantum entanglement via free-space atmosphere link.Comment: 16 pages, 5 figure
Well-posedness and general decay of solution for a transmission problem with viscoelastic term and delay
In this paper, we consider a transmission problem in a bounded domain with a
viscoelastic term and a delay term. Under appropriate hypothesis on the
relaxation function and the relationship between the weight of the damping and
the weight of the delay, we prove the well-posedness result by using
Faedo-Galerkin method. By introducing suitable Lyaponov functionals, we
establish a general decay result, from which the exponential and polynomial
types of decay are only special cases
Laser Driven Ultra-compact Undulator for Synchrotron Radiation
Laser wakefield accelerators have emerged as a promising candidate for
compact synchrotron radiation and even x-ray free electron lasers. Today, to
make the electrons emit electromagnetic radiation, the trajectories of laser
wakefield accelerated electrons are deflected by transverse wakefield,
counter-propagating laser field or external permanent magnet insertion device.
Here, we propose a novel type of undulator which has a few hundred microns of
period and tens of Tesla of magnetic field. The undulator consists of a bifilar
capacitor-coil target which sustains strong discharge current that generates
helical magnetic field around the coil axis when irradiated by a high energy
laser. Coupling this undulator with state-of-the-art laser wakefield
accelerators can, simultaneously, produce ultra-bright quasi-monochromatic
x-rays with tunable energy ranging 5-250 keV and optimize the free electron
laser parameter and gain length compared with permanent magnet based undulator.
This concept may pave a path toward ultra-compact synchrotron radiation and
even x-ray free electron lasers.Comment: 12 pages, 7 figure
Langer Modification, Quantization condition and Barrier Penetration in Quantum Mechanics
The WKB approximation plays an essential role in the development of quantum
mechanics and various important results have been obtained from it. In this
paper, we introduce another method, {\it the so-called uniform asymptotic
approximations}, which is an analytical approximation method to calculate the
wave functions of the Schr\"odinger-like equations, and is applicable to
various problems, including cases with poles (singularities) and multiple
turning points. An distinguished feature of the method is that in each order of
the approximations the upper bounds of the errors are given explicitly. By
properly choosing the freedom introduced in the method, the errors can be
minimized, which significantly improves the accuracy of the calculations. A
byproduct of the method is to provide a very clear explanation of the Langer
modification encountered in the studies of the hydrogen atom and harmonic
oscillator. To further test our method, we calculate (analytically) the wave
functions for several exactly solvable potentials of the Schr\"odinger
equation, and then obtain the transmission coefficients of particles over
potential barriers, as well as the quantization conditions for bound states. We
find that such obtained results agree with the exact ones extremely well.
Possible applications of the method to other fields are also discussed.Comment: revtex4-1, 1 figures, and 1 table. Published in Universe 6 (2020) 9
New Insight into Collimated Plasmon Beam: Nondiffracting versus Linear Focusing
We worked out a new group of collimated plasmon beams by the means of
in-plane diffraction with symmetric phase modulation. As the phase type changes
from 1.8 to 1.0, the beam undergoes an interesting evolution from focusing to a
straight line. Upon this, an intuitive diagram was proposed to elucidate the
beam nature and answer the question whether they are nondiffracting or linear
focusing. Based on this diagram, we further achieved a highly designable scheme
to modulate the beam intensity (e.g., 'lossless' plasmon). Our finding holds
remarkable generality and flexibility in beam engineering and would inspire
more intriguing photonic designs
Performance of Network-Assisted Full-Duplex for Cell-Free Massive MIMO
Network assisted full-duplex (NAFD) is a spatial-division duplex technique
for future wireless networks with cell-free massive multiple-input
multiple-output (CF massive MIMO) network, where a large number of remote
antenna units (RAUs), either using half-duplex or full-duplex, jointly support
truly flexible duplex including time-division duplex, frequency-division duplex
and full duplex on demand of uplink and downlink traffic by using network MIMO
methods. With NAFD, bi-directional data rates of the wireless network could be
increased and end-to-end delay could be reduced. In this paper, the spectral
efficiency of NAFD communications in CF massive MIMO network with imperfect
channel state information (CSI) is investigated under spatial correlated
channels. Based on large dimensional random matrix theory, the deterministic
equivalents for the uplink sum-rate with minimum-mean-square-error (MMSE)
receiver as well as the downlink sum-rate with zero-forcing (ZF) and
regularized zero-forcing (RZF) beamforming are derived. Numerical results show
that under various environmental settings, the deterministic equivalents are
accurate in both a large-scale system and system with a finite number of
antennas. It is also shown that with the downlink-to-uplink interference
cancellation, the uplink spectral efficiency of CF massive MIMO with NAFD could
be improved. The spectral efficiencies of NAFD with different duplex
configurations such as in-band full-duplex, and half-duplex are compared. With
the same total numbers of transmit and receive antennas, NAFD with half-duplex
RAUs offers a higher spectral efficiency. To alleviate the uplink-to-downlink
interference, a novel genetic algorithm based user scheduling strategy (GAS) is
proposed. Simulation results show that the achievable downlink sum-rate by
using the GAS is greatly improved compared to that by using the random user
scheduling
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