109 research outputs found

### Optical properties of photonic crystal slabs with asymmetrical unit cell

Using the unitarity and reciprocity properties of the scattering matrix, we
analyse the symmetry and resonant optical properties of the photonic crystal
slabs (PCS) with complicated unit cell. We show that the reflectivity is not
changed upon the 180deg-rotation of the sample around the normal axis, even in
PCS with asymmetrical unit cell. Whereas the transmissivity becomes
asymmetrical if the diffraction or absorption are present. The PCS reflectivity
peaks to unity near the quasiguided mode resonance for normal light incidence
in the absence of diffraction, depolarisation, and absorptive losses. For the
oblique incidence the full reflectivity is reached only in symmetrical PCS.Comment: 5 pages, 2 Postscript figure

### Gravitational Cherenkov losses in MOND theories

Survival of high-energy cosmic rays (HECRs) against gravitational Cherenkov
losses is shown not to cast strong constraints on MOND theories that are
compatible with general relativity (GR): theories that coincide with GR in the
high-acceleration limit. The energy-loss rate, L, is shown to be many orders
smaller than those derived in the literature for theories with no extra scale.
The gravitational acceleration produced by a HECR in its vicinity is much
higher than the MOND acceleration a0. So, modification to GR, which underlies
L, enters only beyond the MOND radius of the particle, within which GR holds
sway: r_M=sqrt(Gp/c a0). The spectral cutoff, which enters L quadratically, is
thus 1/r_M, not the particle's, much larger, de Broglie wavenumber: k_{dB}=
p/hbar. Thus, L is smaller than published rates, which use k_{dB}, by a factor
(r_M k_{dB})^2~10^{39}(cp/3.10^{11}Gev)^3. With 1/r_M as cutoff, the distance a
HECR can travel without major losses is q l_M, where l_M=c^2/a0 is the MOND
length, and q is a dimensionless function of parameters of the problem. Since
l_M is ~2 pi times the Hubble distance, survival of HECRs does not strongly
constrain GR-compatible, MOND theories. Such theories also easily satisfy
existing preferred-frame limits, inasmuch as these limits are gotten in
high-acceleration systems. I exemplify the results with MOND adaptations of
Einstein-Aether theories.Comment: Phys. Rev. Lett.; 4 pages; added some clarifications and reference

### Diffraction radiation from a screen of finite conductivity

An exact solution has been found for the problem of diffraction radiation
appearing when a charged particle moves perpendicularly to a thin finite screen
having arbitrary conductivity and frequency dispersion. Expressions describing
the Diffraction and Cherenkov emission mechanisms have been obtained for the
spectral-angular forward and backward radiation densities.Comment: 6 pages, 4 figure

### Gravitational diffraction radiation

We show that if the visible universe is a membrane embedded in a
higher-dimensional space, particles in uniform motion radiate gravitational
waves because of spacetime lumpiness. This phenomenon is analogous to the
electromagnetic diffraction radiation of a charge moving near to a metallic
grating. In the gravitational case, the role of the metallic grating is played
by the inhomogeneities of the extra-dimensional space, such as a hidden brane.
We derive a general formula for gravitational diffraction radiation and apply
it to a higher-dimensional scenario with flat compact extra dimensions.
Gravitational diffraction radiation may carry away a significant portion of the
particle's initial energy. This allows to set stringent limits on the scale of
brane perturbations. Physical effects of gravitational diffraction radiation
are briefly discussed.Comment: 5 pages, 2 figures, RevTeX4. v2: References added. Version to appear
in Phys. Rev.

### Experimental Research of the Diffraction and Vavilov-Cherenkov Radiation Generation in a Teflon Target

Geometry of Vavilov-Cherekov (VChR) radiation when an electron moves close to
a dielectric target is in analogy to diffraction radiation (DR) geometry. In
this case we may expect DR generation from the upstream face of the target
besides that VChR. The joint observation of these booth types of radiation is
very interesting from the pseudo-photon viewpoint, which is applicable for
relativistic electrons. Unexpected results obtained in our experiment insist on
reflection about nature both DR and VChR. The experiment was performed on the
relativistic electron beam of the microtron of Tomsk Polytechnic University.Comment: This article will be published in Journal of Physic

### Self-amplified Cherenkov radiation from a relativistic electron in a waveguide partially filled with a laminated material

The radiation from a relativistic electron uniformly moving along the axis of
cylindrical waveguide filled with laminated material of finite length is
investigated. Expressions for the spectral distribution of radiation passing
throw the transverse section of waveguide at large distances from the laminated
material are derived with no limitations on the amplitude and variation profile
of the layered medium permittivity and permeability. Numerical results for
layered material consisting of dielectric plates alternated with vacuum gaps
are given. It is shown that at a special choice of problem parameters,
Cherenkov radiation generated by the relativistic electron inside the plates is
self-amplified. The visual explanation of this effect is given and a possible
application is discussed.Comment: 8 pages, 4 figures,1 table, the paper is accepted for publication in
the Journal of Physics: Conference Serie

### Fermat's principle of least time in the presence of uniformly moving boundaries and media

The refraction of a light ray by a homogeneous, isotropic and non-dispersive
transparent material half-space in uniform rectilinear motion is investigated
theoretically. The approach is an amalgamation of the original Fermat's
principle and the fact that an isotropic optical medium at rest becomes
optically anisotropic in a frame where the medium is moving at a constant
velocity. Two cases of motion are considered: a) the material half-space is
moving parallel to the interface; b) the material half-space is moving
perpendicular to the interface. In each case, a detailed analysis of the
obtained refraction formula is provided, and in the latter case, an intriguing
backward refraction of light is noticed and thoroughly discussed. The results
confirm the validity of Fermat's principle when the optical media and the
boundaries between them are moving at relativistic speeds.Comment: 11 pages, 6 figures, RevTeX 4, comments welcome; V2: revised, Fig. 7
added; V3: several typos corrected, accepted for publication in European
Journal of Physics (online at: http://stacks.iop.org/EJP/28/933

### Space-Time Evolution of Ultrarelativistic Quantum Dipoles in Quantum Electrodynamics

We discuss space-time evolution of ultrarelativistic quantum dipole in QED.
We show that the space-time evolution can be described, in a certain
approximation, by means of a regularized wave function, whose parameters are
determined by the process of the dipole creation by a local current. We derive
using these wave functions the dipole expansion law, that is found to coincide
parametrically in the leading order with the one suggested by Farrar,
Frankfurt,Liu and Strikman.Comment: 15 page

### Coherent Cherenkov radiation as an intense THz source

Diffraction and Cherenkov radiation of relativistic electrons from a dielectric target has been proposed as mechanism for production of intense terahertz (THz) radiation. The use of an extremely short high-energy electron beam of a 4th generation light source (X-ray free electron laser) appears to be very promising. A moderate power from the electron beam can be extracted and converted into THz radiation with nearly zero absorption losses. The initial experiment on THz observation will be performed at CLARA/VELA FEL test facility in the UK to demonstrate the principle to a wider community and to develop the radiator prototype. In this paper, we present our theoretical predictions (based on the approach of polarization currents), which provides the basis for interpreting the future experimental measurements. We will also present our hardware design and discuss a plan of the future experiment

### Some features of electromagnetic field of charged particle revolving about dielectric ball

A relativistic electron uniformly rotating along an equatorial orbit around a
dielectric ball may generate Cherenkov radiation tens of times more intense as
that in case of revolution of a particle in a continuous, infinite and
transparent medium. The root-mean-square values of electric and magnetic field
strengths of particle are practically not localized in the central part of the
equatorial plane of ball and close to the poles of ball.Comment: 6 pages, 3 figures, contribution to Proceedings of International
Symposium RREPS-2009, 07-11 September, 2009, Zvenigorod, Russi

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