644 research outputs found
Scintillation reduction for combined Gaussian-vortex beam propagating through turbulent atmosphere
We numerically examine the spatial evolution of the structure of coherent and
partially coherent laser beams (PCBs), including the optical vortices,
propagating in turbulent atmospheres. The influence of beam fragmentation and
wandering relative to the axis of propagation (z-axis) on the value of the
scintillation index (SI) of the signal at the detector is analyzed. A method
for significantly reducing the SI, by averaging the signal at the detector over
a set of PCBs, is described. This novel method is to generate the PCBs by
combining two laser beams - Gaussian and vortex beams, with different
frequencies (the difference between these two frequencies being significantly
smaller than the frequencies themselves). In this case, the SI is effectively
suppressed without any high-frequency modulators.Comment: 13 pages, 8 figure
Reduction of Magnetic Noise in Magnetic Resonance Force Microscopy
We study the opportunity to reduce a magnetic noise produced by a uniform
cantilever with a ferromagnetic particle in magnetic resonance force microscopy
(MRFM) applications. We demonstrate theoretically a significant reduction of
magnetic noise and the corresponding increase of the MRFM relaxation time using
a nonuniform cantilever
Spin Diffusion and Relaxation in a Nonuniform Magnetic Field
We consider a quasiclassical model that allows us to simulate the process of
spin diffusion and relaxation in the presence of a highly nonuniform magnetic
field. The energy of the slow relaxing spins flows to the fast relaxing spins
due to the dipole-dipole interaction between the spins. The magnetic field
gradient suppresses spin diffusion and increases the overall relaxation time in
the system. The results of our numerical simulations are in a good agreement
with the available experimental data.Comment: 11 pages and 6 figure
Beam Wandering in the Atmosphere: The Effect of Partial Coherence
The effect of a random phase screen on laser beam wander in a turbulent
atmosphere is studied theoretically. The method of photon distribution function
is used to describe the photon kinetics of both weak and strong turbulence. By
bringing together analytical and numerical calculations, we have obtained the
variance of beam centroid deflections caused by scattering on turbulent eddies.
It is shown that an artificial distortion of the initial coherence of the
radiation can be used to decrease the wandering effect. The physical mechanism
responsible for this reduction and applicability of our approach are discussed.Comment: 16 pages, 5 figure
Regular and Random Magnetic Resonance Force Microscopy Signal with a Cantilever Oscillating Parallel to a Sample Surface
We study theoretically the magnetic resonance force microscopy (MRFM) in
oscillating cantilever-driven adiabatic reversals (OSCAR) technique, for the
case when the cantilever tip oscillates parallel to the surface of a sample.
The main contribution to the MRFM signal is associated with a part of the
resonance slice near the surface of the sample. The regular (approximately
exponential) decay of the MRFM signal is followed by the non-dissipating random
signal. The Fourier spectrum of the random signal has a characteristic peak
which can be used for the identification of the signal.Comment: 9 pages, 5 figure
Production of a pion in association with a high-Q2 dilepton pair in antiproton-proton annihilation at GSI-FAIR
We evaluate the cross section for anti-p p -> l+ l- pi0 in the forward
direction and for large lepton pair invariant mass. In this kinematical region,
the leading-twist amplitude factorises into a short-distance matrix element,
long-distance dominated antiproton Distribution Amplitudes and proton to pion
Transition Distribution Amplitudes (TDA). Using a modelling inspired from the
chiral limit for these TDAs, we obtain a first estimate of this cross section,
thus demonstrating that this process can be measured at GSI-FAIR.Comment: Latex, 5 pages, 3 figure
Heat engines and heat pumps in a hydrostatic atmosphere: How surface pressure and temperature constrain wind power output and circulation cell size
The kinetic energy budget of the atmosphere's meridional circulation cells is
analytically assessed. In the upper atmosphere kinetic energy generation grows
with increasing surface temperature difference \$\Delta T_s\$ between the cold
and warm ends of a circulation cell; in the lower atmosphere it declines. A
requirement that kinetic energy generation is positive in the lower atmosphere
limits the poleward cell extension \$L\$ of Hadley cells via a relationship
between \$\Delta T_s\$ and surface pressure difference \$\Delta p_s\$: an upper
limit exists when \$\Delta p_s\$ does not grow with increasing \$\Delta T_s\$.
This pattern is demonstrated here using monthly data from MERRA re-analysis.
Kinetic energy generation along air streamlines in the boundary layer does not
exceed \$40\$~J~mol\$^{-1}\$; it declines with growing \$L\$ and reaches zero
for the largest observed \$L\$ at 2~km height. The limited meridional cell size
necessitates the appearance of heat pumps -- circulation cells with negative
work output where the low-level air moves towards colder areas. These cells
consume the positive work output of the heat engines -- cells where the
low-level air moves towards the warmer areas -- and can in theory drive the
global efficiency of atmospheric circulation down to zero. Relative
contributions of \$\Delta p_s\$ and \$\Delta T_s\$ to kinetic energy generation
are evaluated: \$\Delta T_s\$ dominates in the upper atmosphere, while \$\Delta
p_s\$ dominates in the lower. Analysis and empirical evidence indicate that the
net kinetic power output on Earth is dominated by surface pressure gradients,
with minor net kinetic energy generation in the upper atmosphere. The role of
condensation in generating surface pressure gradients is discussed.Comment: 26 pages, 9 figures, 2 tables; re-organized presentation, more
discussion and a new figure (Fig. 4) added; in Fig. 3 the previously
invisible dots (observations) can now be see
Photon storage in Lambda-type optically dense atomic media. II. Free-space model
In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)], we
presented a universal physical picture for describing a wide range of
techniques for storage and retrieval of photon wave packets in Lambda-type
atomic media in free space, including the adiabatic reduction of the photon
group velocity, pulse-propagation control via off-resonant Raman techniques,
and photon-echo based techniques. This universal picture produced an optimal
control strategy for photon storage and retrieval applicable to all approaches
and yielded identical maximum efficiencies for all of them. In the present
paper, we present the full details of this analysis as well some of its
extensions, including the discussion of the effects of non-degeneracy of the
two lower levels of the Lambda system. The analysis in the present paper is
based on the intuition obtained from the study of photon storage in the cavity
model in the preceding paper [Gorshkov et al., Phys. Rev. A 76, 033804 (2007)].Comment: 26 pages, 8 figures. V2: significant changes in presentation, new
references, higher resolution of figure
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