20,477 research outputs found
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
Quantum diffusion of dipole-oriented indirect excitons in coupled quantum wells
A model for diffusion of statistically-degenerate excitons in (coupled)
quantum wells is proposed and analysed. Within a microscopic approach, we
derive a quantum diffusion equation, calculate and estimate the self-diffusion
coefficient for excitons in quantum wells and derive a modified Einstein
relation adapted to statistically-degenerated quasi-two-dimensional bosons. It
is also shown that the dipole-dipole interaction of indirect excitons
effectively screens long-range-correlated disorder in quantum wells. Numerical
calculations are given for indirect excitons in GaAs/AlGaAs coupled quantum
wells.Comment: To appear in Europhysics Letter
Vortex solitons in dispersive nonlinear Kerr type media
We have investigated the nonlinear amplitude vector equation governing the
evolution of optical pulses in optical and UV region. We are normalizing this
equation for the cases of different and equal transverse and longitudinal size
of optical pulses, of week and strong dispersion. This gives us the possibility
to reduce the amplitude equation to different nonlinear evolution equations in
the partial cases. For some of these nonlinear equations exact vortex solutions
are found. Conditions for experimental observations of these vortices are
determined.Comment: 28 pages, 9 figures, Late
Statistical characteristics of irreversible predictability time in regional ocean models
Probabilistic aspects of regional ocean model predictability is analyzed using the probability density function (PDF) of the irreversible predictability time (IPT) (called τ-PDF) computed from an unconstrained ensemble of stochastic perturbations in initial conditions, winds, and open boundary conditions. Two-attractors (a chaotic attractor and a small-amplitude stable limit cycle) are found in the wind-driven circulation. Relationship between attractor's residence time and IPT determines the τ-PDF for the short (up to several weeks) and intermediate (up to two months) predictions. The τ-PDF is usually non-Gaussian but not multi-modal for red-noise perturbations in initial conditions and perturbations in the wind and open boundary conditions. Bifurcation of τ-PDF occurs as the tolerance level varies. Generally, extremely successful predictions (corresponding to the τ-PDF's tail toward large IPT domain) are not outliers and share the same statistics as a whole ensemble of predictions
On the polarization properties of the charmed baryon Lambda^+_c in the Lambda^+_c -> p + K^- + pi^+ + pi^0 decay
The polarization properties of the charmed Lambda^+_c baryon are investigated
in weak non-leptonic four-body Lambda^+_c -> p + K^- + pi^+ + pi^0 decay. The
probability of this decay and the angular distribution of the probability are
calculated in the effective quark model with chiral U(3)XU(3) symmetry
incorporating Heavy Quark Effective theory (HQET) and the extended
Nambu-Jona-Lasinio model with a linear realization of chiral U(3)XU(3)
symmetry. The theoretical value of the probability of the decay Lambda^+_c -> p
+ K^- + pi^+ + pi^0 relative to the probability of the decay Lambda^+_c -> p +
K^- + pi^+ does not contain free parameters and fits well experimental data.
The application of the obtained results to the analysis of the polarization of
the Lambda^+_c produced in the processes of photo and hadroproduction is
discussed.Comment: 10 pages, no figures, Late
Comments on the Background Field Method in Harmonic Superspace: Non-holomorphic Corrections in N=4 SYM
We analyse the one-loop effective action of N=4 SYM theory in the framework
of the background field formalism in N=2 harmonic superspace. For the case of
on-shell background N=2 vector multiplet we prove that the effective action is
free of harmonic singularities. When the lowest N=1 superspace component of the
N=2 vector multiplet is switched off, the effective action of N=4 SYM theory is
shown to coincide with that obtained by Grisaru et al on the base of the N=1
background field method. We compute the leading non-holomorphic corrections to
the N=4 SU(2) SYM effective action.Comment: 15 pages, latex, no figures, some comments adde
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