1,641 research outputs found
Super pulses of orbital angular momentum in fractional-order spiroid vortex-beams
We consider optical properties of Hypergeometric-Gaussian beam compositions
with spiral-like intensity and phase distributions that were called the spiroid
beams. Their orbital angular momentum as a function of a fractional-order
topological charge has a chain of supper-pulses (bursts and dips).The form of
the supper-pulses can be controlled by the spiral parameters. Such a phenomenon
can be used in optical switches and triggers for optical devices and
communication systems.Comment: 4 pages, 7 figure
Optical orientation of nuclei in nitrogen alloys GaAsN at room temperature
The intensity and the giant circular polarization of edge luminescence in a
longitudinal magnetic field have been measured in nitrogen alloys GaAsN under
circularly polarized pumping. It has been found that these dependences are
shifted with respect to zero field by a value Beff. The magnitude of the
internal field Beff increases with increase in pumping intensity and reaches
saturation (~250 Gauss) at great densities of excitation. The saturation of the
Beff field with growth of pumping indicates that this is a field of nuclei,
polarized dynamically due to hyperfine interaction with optically oriented deep
paramagnetic centers, rather than a field of exchange interaction created on
the center by spin-polarized photo-excited conduction electrons. The short time
of nuclear polarization by electrons (<15 mks), measured under modulation of
circular polarization of the exciting light with high frequency, points to a
small number of nuclei undergoing hyperfine interaction with an electron
localized at a center.Comment: 6 pages, 2 figures, Accepted by JETP Letter
The avalanche instability of the orbital angular momentum in combined vortex beams
We presented a new method for measuring the squares of the amplitudes and
phases of partial vortex-beams in a complex beam array in real time. The method
is based on measuring the high-order intensity moments and analyzing the
solutions of a system of linear equations. Calibration measurements have shown
that the measurement error at least for an array of 10-15 beams does not exceed
4%. It has theoretically and experimentally been shown that the orbital angular
momentum (OAM) of combined singular beams depends essentially on the fractional
topological charge p. For integer values of the topological charge, the OAM
obtains the maximum value numerically equal to the topological charge of the
singular beam. With weak deviations of the topological charge in the direction
of fractional values, dips in the orbital moment appear. . It was
experimentally shown that even a weak perturbation of the holographic grating
leads to a sharp increase in the contribution of partial beams with other
integer-order topological charges.Comment: 17 pages, 13 figure
Microwave Generation Power in a Nonrelativistic Electron Beam with Virtual Cathode in a Retarding Electric Field
The power of microwave generation in a nonrelativistic electron beam with
virtual cathode formed in a static retarding electric field (low-voltage
vircator system) has been studied experimentally and by means of numerical
simulation within the framework of a one-dimensional theory. The limits of
applicability of the one-dimensional theory have been experimentally
determined.Comment: 4 pages, 2 figure
Quadrefringence of optical vortices in a uniaxial crystal
The splitting of a single optical vortex into four separate ones in a
singular beam is theoretically and experimentally described for the propagation
of light obliquely through a uniaxial crystal. Also we found the condition
under which the new-born vortices in each four individual beams propagate
independently without dislocation reactions and have different locations in all
beams for any crystal lengths.Comment: 10 pages, 8 figure
Nonlinear Bloch-waves and current states of exciton-polariton condensates
The formation of nonlinear Bloch states in open driven-dissipative system of
exciton-polaritons loaded into a weak-contrast 1D periodic lattice is studied
numerically and analytically. The condensate is described within the framework
of mean-field theory by the coupled equations for the order parameter and for
the density of incoherent excitons. The stationary nonlinear solutions having
the structure of Bloch waves are studied in detail. It is shown that there is a
bifurcation leading to the appearance of a family of essentially nonlinear
states. The special feature of these solutions is that its current does not
vanish when the quasi-momentum of the state approaches the values equal to the
half of the lattice constant. To explain the bifurcations found in numerical
simulations a simple perturbative approach is developed. The stability of the
nonlinear states is examined by linear spectral analysis and by direct
numerical simulations. An experimental scheme allowing the observation of the
discussed nonlinear current states is suggested and studied by numerical
simulations.Comment: 12 pages, 7 figure
Structure and Kinematics of the Interstellar Medium in the Star-Forming Region in the BCD Galaxy VIIZw403 (UGC6456)
The structure and kinematics of ionized gas in the star-forming region in the
BCD galaxy VIIZw403 (UGC6456) are analyzed using observations with the SCORPIO
focal reducer on the 6-m Special Astrophysical Observatory telescope in three
modes: direct imaging (in the H-alpha, [OIII], and [SII] lines), long-slit
spectroscopy, and spectroscopy with a scanning Fabry-Perot interferometer. In
addition to the previously known bright HII regions and the faint giant ring
that surrounds the entire star-forming region, many new faint diffuse and arc
structures have been detected. Fine structure of the giant ring has been
revealed. We do not confirm the previously detected expansion of the bright
shells around young stellar associations with a velocity of 50-70 km/s. We have
estimated their expansion velocities to be no higher than 15-20 km/s; the
corresponding kinematic age, no younger than 3-4 Myr, agrees well with the age
of the compact OB associations related to them. We associate the faint extended
filamentary and diffuse regions of ionized gas identified in nearly the entire
central region of the galaxy and the giant HII ring with the older (10 Myr)
stellar population of the most recent starburst. Weak high-velocity [OIII] and
H-alpha line wings (up to 300 km/s from the line center) have been detected in
the brightest HII region. Such velocities have been observed in the galaxy for
the first time. The previously published H-alpha luminosity measurements for
the galaxy are refined.Comment: to appear in Astronomy Letters, 13 pages, 6 postscript figures, 1 in
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With no Color and Scent (part II): Metal and Alloy Microstructures-Handmade Replicas of Natural Objects
As a continuation of work on metal and alloy "plants" synthesis on porous
membranes by means of pulsed current electroplating volume metallic
microstructures resembling such natural objects as shells, cabbage leaves,
mushrooms are grown and presented in their modest elegance. Such structures are
formed from PdNi and PdCo alloys as well as Ag, Cu and Ni in conditions defined
by the shape of membrane pores and the parameters of the pulsed current. It is
shown that the obtained complex structures are formed by layers of metallic
nanowires as a result of their self-assembly while growing during the pulsed
current electroplating process. Depending on the shape of the membrane and the
regime of the pulsed current electroplating either one type of shell-like
structures or various structures can be grown.Comment: 11pages, 10 figure
Experimental and Theoretical Investigations of Stochastic Oscillatory Phenomena in a Nonrelativistic Electron Beam with a Virtual Cathode
Results are presented from experimental investigations of oscillatory
phenomena in an electron beam with a virtual cathode in a diode gap with a
decelerating field. Experiments have revealed a stochastic broadband generation
of the microwave oscillations of a virtual cathode in a decelerating field.
Numerical simulations based on a simple one-dimensional model have shown that
the onset of the stochastic generation and the broadening of the oscillation
spectrum with increasing beam deceleration rate are governed by the processes
of regrouping of the electrons in a beam with a virtual cathode.Comment: 15 pages, 12 figure
Oscillatory dynamics of non-equilibrium dissipative exciton-polariton condensates in weak-contrast lattices
We study nonlinear dynamics of exciton-polaritons in an incoherently pumped
semiconductor microcavity with embedded weak-contrast lattice and coupled to an
exciton reservoir. We elucidate fundamental features of non-equilibrium
exciton-polariton condensate trapped in one-dimensional periodical potential
close to zero momentum (so-called "Zero-state") and to the state at the
boundary of Brillouin zone ("-state"). Within the framework of the
mean-field theory, we identify different regimes of both relaxation and
oscillatory dynamics of coherent exciton-polaritons governed by superpositions
of Bloch eigenstates within the periodic lattice. In particular, we
theoretically demonstrate stable macroscopical oscillations, akin to nonlinear
Josephson oscillations, between different spectral components of a polariton
condensate in the momenta-space. We elucidate a strong influence of the
dissipative effects and the feedback induced by the inhomogeneity of incoherent
reservoir on the dynamics of the coherent polaritons.Comment: 9 pages, 5 figure
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