117 research outputs found
Bleak media scenarios confront Vanuatu
Lack of qualified and experienced journalists, a tendency to allow censorship and disregard for the notion of publicly funding broadcasting services belonging to the public hamper freedom of information. 
Nonlinear resonance reflection from and transmission through a dense glassy system built up of oriented linear Frenkel chains: two-level models
A theoretical study of the resonance optical response of assemblies of
oriented short (as compared to an optical wavelength) linear Frenkel chains is
carried out using a two-level model. We show that both transmittivity and
reflectivity of the film may behave in a bistable fashion and analyze how the
effects found depend on the film thickness and on the inhomogeneous width of
the exciton optical transition.Comment: 26 pages, 9 figure
Stable spinning optical solitons in three dimensions
We introduce spatiotemporal spinning solitons (vortex tori) of the
three-dimensional nonlinear Schrodinger equation with focusing cubic and
defocusing quintic nonlinearities. The first ever found completely stable
spatiotemporal vortex solitons are demonstrated. A general conclusion is that
stable spinning solitons are possible as a result of competition between
focusing and defocusing nonlinearities.Comment: 4 pages, 6 figures, accepted to Phys. Rev. Let
Synergistic Effects in Two-Photon Absorption: the Quantum Electrodynamics of Bimolecular Mean-Frequency Absorption
Nonlinear theory of soliton-induced waveguides
We develop a nonlinear theory of soliton-induced waveguides that describes a
finite-amplitude probe beam guided by a spatial dark soliton in a saturable
nonlinear medium. We suggest an effective way to control the interaction of
these soliton-induced waveguides and also show that, in sharp contrast with
scalar dark solitons, the dark-soliton waveguides can attract each other and
even form stationary bound states.Comment: 3 pages, 4 figure
Theory of radiation trapping by the accelerating solitons in optical fibers
We present a theory describing trapping of the normally dispersive radiation
by the Raman solitons in optical fibers. Frequency of the radiation component
is continuously blue shifting, while the soliton is red shifting. Underlying
physics of the trapping effect is in the existence of the inertial gravity-like
force acting on light in the accelerating frame of reference. We present
analytical calculations of the rate of the opposing frequency shifts of the
soliton and trapped radiation and find it to be greater than the rate of the
red shift of the bare Raman soliton. Our findings are essential for
understanding of the continuous shift of the high frequency edge of the
supercontinuum spectra generated in photonic crystal fibers towards higher
frequencies.Comment: Several misprints in text and formulas corrected. 10 pages, 9
figures, submitted to Phys. Rev.
Anomalous Momentum States, Non-Specular Reflections, and Negative Refraction of Phase-Locked, Second Harmonic Pulses
We simulate and discuss novel spatio-temporal propagation effects that relate
specifically to pulsed, phase-mismatched second harmonic generation in a
negative index material having finite length. Using a generic Drude model for
the dielectric permittivity and magnetic permeability, the fundamental and
second harmonic frequencies are tuned so that the respective indices of
refraction are negative for the pump and positive for the second harmonic
signal. A phase-locking mechanism causes part of the second harmonic signal
generated at the entry surface to become trapped and dragged along by the pump
and to refract negatively, even though the index of refraction at the second
harmonic frequency is positive. These circumstances culminate in the creation
of an anomalous state consisting of a forward-moving second harmonic wave
packet that has negative wave vector and momentum density, which in turn leads
to non-specular reflections at intervening material interfaces. The
forward-generated second harmonic signal trapped under the pump pulse
propagates forward, but has all the attributes of a reflected pulse, similar to
its twin counterpart generated at the surface and freely propagating backward
away from the interface. This describes a new state of negative refraction,
associated with nonlinear frequency conversion and parametric processes,
whereby a beam generated at the interface can refract negatively even though
the index of refraction at that wavelength is positive
On the Properties of Two Pulses Propagating Simultaneously in Different Dispersion Regimes in a Nonlinear Planar Waveguide
Properties of two pulses propagating simultaneously in different dispersion
regimes, anomalous and normal, in a Kerr-type planar waveguide are studied in
the framework of the nonlinear Schroedinger equation. Catastrophic
self-focusing and spatio-temporal splitting of the pulses is investigated. For
the limiting case when the dispersive term of the pulse propagating in the
normal dispersion regime can be neglected an indication of a possibility of a
stable self-trapped propagation of both pulses is obtained.Comment: 18 pages (including 15 eps figures
Spontaneous emission and level shifts in absorbing disordered dielectrics and dense atomic gases: A Green's function approach
Spontaneous emission and Lamb shift of atoms in absorbing dielectrics are
discussed. A Green's-function approach is used based on the multipolar
interaction Hamiltonian of a collection of atomic dipoles with the quantised
radiation field. The rate of decay and level shifts are determined by the
retarded Green's-function of the interacting electric displacement field, which
is calculated from a Dyson equation describing multiple scattering. The
positions of the atomic dipoles forming the dielectrics are assumed to be
uncorrelated and a continuum approximation is used. The associated unphysical
interactions between different atoms at the same location is eliminated by
removing the point-interaction term from the free-space Green's-function (local
field correction). For the case of an atom in a purely dispersive medium the
spontaneous emission rate is altered by the well-known Lorentz local-field
factor. In the presence of absorption a result different from previously
suggested expressions is found and nearest-neighbour interactions are shown to
be important.Comment: 6 pages no figure
Observation of relativistic cross-phase modulation in high-intensity laser-plasma interactions
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