39,500 research outputs found
Negative phase velocity in nonlinear oscillatory systems --mechanism and parameter distributions
Waves propagating inwardly to the wave source are called antiwaves which have
negative phase velocity. In this paper the phenomenon of negative phase
velocity in oscillatory systems is studied on the basis of periodically paced
complex Ginzbug-Laundau equation (CGLE). We figure out a clear physical picture
on the negative phase velocity of these pacing induced waves. This picture
tells us that the competition between the frequency of the
pacing induced waves with the natural frequency of the oscillatory
medium is the key point responsible for the emergence of negative phase
velocity and the corresponding antiwaves. and
are the criterions for the waves with negative
phase velocity. This criterion is general for one and high dimensional CGLE and
for general oscillatory models. Our understanding of antiwaves predicts that no
antispirals and waves with negative phase velocity can be observed in excitable
media
Probing vacuum birefringence by phase-contrast Fourier imaging under fields of high-intensity lasers
In vacuum high-intensity lasers can cause photon-photon interaction via the
process of virtual vacuum polarization which may be measured by the phase
velocity shift of photons across intense fields. In the optical frequency
domain, the photon-photon interaction is polarization-mediated described by the
Euler-Heisenberg effective action. This theory predicts the vacuum
birefringence or polarization dependence of the phase velocity shift arising
from nonlinear properties in quantum electrodynamics (QED). We suggest a method
to measure the vacuum birefringence under intense optical laser fields based on
the absolute phase velocity shift by phase-contrast Fourier imaging. The method
may serve for observing effects even beyond the QED vacuum polarization.Comment: 14 pages, 9 figures. Accepted by Applied Physics
Plane waves with negative phase velocity in Faraday chiral mediums
The propagation of plane waves in a Faraday chiral medium is investigated.
Conditions for the phase velocity to be directed opposite to the direction of
power flow are derived for propagation in an arbitrary direction; simplified
conditions which apply to propagation parallel to the distinguished axis are
also established. These negative phase-velocity conditions are explored
numerically using a representative Faraday chiral medium, arising from the
homogenization of an isotropic chiral medium and a magnetically biased ferrite.
It is demonstrated that the phase velocity may be directed opposite to power
flow, provided that the gyrotropic parameter of the ferrite component medium is
sufficiently large compared with the corresponding nongyrotropic permeability
parameters.Comment: accepted for publication in Phys. Rev.
Diffraction gratings of isotropic negative phase-velocity materials
Diffraction of electromagnetic plane waves by the gratings made by
periodically corrugating the exposed planar boundaries of homogeneous,
isotropic, linear dielectric--magnetic half--spaces is examined. The phase
velocity vector in the diffracting material can be either co-parallel or
anti-parallel to the time-averaged Poynting vector, thereby allowing for the
material to be classified as of either the positive or the negative negative
phase-velocity (PPV or NPV) type. Three methods used for analyzing dielectric
gratings - the Rayleigh-hypothesis method, a perturbative approach, and the C
formalism - are extended here to encompass NPV gratings by a careful
consideration of field representation inside the refracting half--space.
Corrugations of both symmetric as well as asymmetric shapes are studied, as
also the diversity of grating response to the linear polarization states of the
incident plane wave. The replacement of PPV grating by its NPV analog affects
only nonspecular diffraction efficiencies when the corrugations are shallow,
and the effect on specular diffraction efficiencies intensifies as the
corrugations deepen. Whether the type of the refracting material is NPV or PPV
is shown to affect surface wave propagation as well as resonant excitation of
surface waves.Comment: 28 pages, 10 figures in 27 file
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