15,983 research outputs found
Localized solutions of Lugiato-Lefever equations with focused pump
Lugiato-Lefever (LL) equations in one and two dimensions (1D and 2D)
accurately describe the dynamics of optical fields in pumped lossy cavities
with the intrinsic Kerr nonlinearity. The external pump is usually assumed to
be uniform, but it can be made tightly focused too -- in particular, for
building small pixels. We obtain solutions of the LL equations, with both the
focusing and defocusing intrinsic nonlinearity, for 1D and 2D confined modes
supported by the localized pump. In the 1D setting, we first develop a simple
perturbation theory, based in the sech ansatz, in the case of weak pump and
loss. Then, a family of exact analytical solutions for spatially confined modes
is produced for the pump focused in the form of a delta-function, with a
nonlinear loss (two-photon absorption) added to the LL model. Numerical
findings demonstrate that these exact solutions are stable, both dynamically
and structurally (the latter means that stable numerical solutions close to the
exact ones are found when a specific condition, necessary for the existence of
the analytical solution, does not hold). In 2D, vast families of stable
confined modes are produced by means of a variational approximation and full
numerical simulations.Comment: 26 pages, 9 figures, accepted for publication in Scientific Report
Einstein-Podolsky-Rosen correlations via dissociation of a molecular Bose-Einstein condensate
Recent experimental measurements of atomic intensity correlations through
atom shot noise suggest that atomic quadrature phase correlations may soon be
measured with a similar precision. We propose a test of local realism with
mesoscopic numbers of massive particles based on such measurements. Using
dissociation of a Bose-Einstein condensate of diatomic molecules into bosonic
atoms, we demonstrate that strongly entangled atomic beams may be produced
which possess Einstein-Podolsky-Rosen (EPR) correlations in field quadratures,
in direct analogy to the position and momentum correlations originally
considered by EPR.Comment: Final published version (corrections in Ref. [32], updated
references
Observation of Enhanced Beaming from Photonic Crystal Waveguides
We report on the experimental observation of the beaming effect in photonic
crystals enhanced via surface modes. We experimentally map the spatial field
distribution of energy emitted from a subwavelength photonic crystal waveguide
into free-space, rendering with crisp clarity the diffractionless beaming of
energy. Our experimental data agree well with our numerical studies of the
beaming enhancement in photonic crystals with modulated surfaces. Without loss
of generality, we study the beaming effect in a photonic crystal scaled to
microwave frequencies and demonstrate the technological capacity to deliver
long-range, wavelength-scaled beaming of energy.Comment: 4 pages, 6 figure
The hippocampus and cerebellum in adaptively timed learning, recognition, and movement
The concepts of declarative memory and procedural memory have been used to distinguish two basic types of learning. A neural network model suggests how such memory processes work together as recognition learning, reinforcement learning, and sensory-motor learning take place during adaptive behaviors. To coordinate these processes, the hippocampal formation and cerebellum each contain circuits that learn to adaptively time their outputs. Within the model, hippocampal timing helps to maintain attention on motivationally salient goal objects during variable task-related delays, and cerebellar timing controls the release of conditioned responses. This property is part of the model's description of how cognitive-emotional interactions focus attention on motivationally valued cues, and how this process breaks down due to hippocampal ablation. The model suggests that the hippocampal mechanisms that help to rapidly draw attention to salient cues could prematurely release motor commands were not the release of these commands adaptively timed by the cerebellum. The model hippocampal system modulates cortical recognition learning without actually encoding the representational information that the cortex encodes. These properties avoid the difficulties faced by several models that propose a direct hippocampal role in recognition learning. Learning within the model hippocampal system controls adaptive timing and spatial orientation. Model properties hereby clarify how hippocampal ablations cause amnesic symptoms and difficulties with tasks which combine task delays, novelty detection, and attention towards goal objects amid distractions. When these model recognition, reinforcement, sensory-motor, and timing processes work together, they suggest how the brain can accomplish conditioning of multiple sensory events to delayed rewards, as during serial compound conditioning.Air Force Office of Scientific Research (F49620-92-J-0225, F49620-86-C-0037, 90-0128); Advanced Research Projects Agency (ONR N00014-92-J-4015); Office of Naval Research (N00014-91-J-4100, N00014-92-J-1309, N00014-92-J-1904); National Institute of Mental Health (MH-42900
Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport
Excitation transfer through interacting systems plays an important role in
many areas of physics, chemistry, and biology. The uncontrollable interaction
of the transmission network with a noisy environment is usually assumed to
deteriorate its transport capacity, especially so when the system is
fundamentally quantum mechanical. Here we identify key mechanisms through which
noise such as dephasing, perhaps counter intuitively, may actually aid
transport through a dissipative network by opening up additional pathways for
excitation transfer. We show that these are processes that lead to the
inhibition of destructive interference and exploitation of line broadening
effects. We illustrate how these mechanisms operate on a fully connected
network by developing a powerful analytical technique that identifies the
invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we
show how these principles can explain the remarkable efficiency and robustness
of excitation energy transfer from the light-harvesting chlorosomes to the
bacterial reaction center in photosynthetic complexes and present a numerical
analysis of excitation transport across the Fenna-Matthew-Olson (FMO) complex
together with a brief analysis of its entanglement properties. Our results show
that, in general, it is the careful interplay of quantum mechanical features
and the unavoidable environmental noise that will lead to an optimal system
performance.Comment: 16 pages, 9 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/09014454 . New revised version;
discussion of entanglement properties enhance
Experimental evaluation of signal-to-noise in spectro-holography via modified uniformly redundant arrays in the soft x-ray and extreme ultraviolet spectral regime
We present dichroic x-ray lensless magnetic imaging by Fourier transform holography with an extended reference scheme via a modified uniformly redundant array (mURA). Holographic images of magnetic domains simultaneously generated by a single pinhole reference as well as by a mURA reference are compared with respect to the signal-to-noise ratio (SNR) as a function of exposure time. We apply this approach for spectro-holographic imaging of ferromagnetic domain patterns in Co/Pt multilayer films. Soft x-rays with wavelengths of 1.59 nm (Co L 3 absorption edge) and 20.8 nm (Co M 2,3 absorption edges) are used for image formation and to generate contrast via x-ray magnetic circular dichroism. For a given exposure time, the mURA-based holography allows to decouple the reconstruction SNR from the spatial resolution. For 1.59 nm wavelength, the reconstruction via the extended reference scheme shows no significant loss of spatial resolution compared to the single pinhole reference. In contrast, at 20.8 nm wavelength the single pinhole reveals some very intricate features which are lost in the image generated by the mURA, although overall a high-quality image is generated. The SNR-advantage of the mURA scheme is most notable when the hologram has to be encoded with few photons, while errors associated with the increased complexity of the reconstruction process reduce the advantage for high-photon-number experiments.BMBF, 05K13KT3, Verbundprojekt 05K2013 - DynaMaX: Messplatz für ultraschnelle Dynamik bei BESSY II. Teilprojekt
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