6,709 research outputs found
Weak-wave advancement in nearly collinear four-wave mixing
We identify a new four-wave mixing process in which two nearly collinear pump
beams produce phase-dependent gain into a weak bisector signal beam in a
self-defocusing Kerr medium. Phase matching is achieved by weak-wave
advancement caused by cross-phase modulation between the pump and signal beams.
We relate this process to the inverse of spatial modulational instability and
suggest a time-domain analog.Comment: 7 pages, 3 figure
Data acquisition system for NASA LaRC impact dynamics research facility
A data system is designed to permit the simultaneous recording of 90 data channels on one 28 track magnetic tape recorder using a constant bandwidth FM multiplexing technique. Dynamic signals from transducers located in the test aircraft are amplified and fed to voltage controlled oscillators where they are converted to discrete FM signals. The signals from each group of five VCO's are fed to a mixer/distribution amplifier where they are combined into one composite signal and recorded, using direct recording techniques, on one magnetic tape recorder track. Millivolt signals from the recorders reproduce heads are amplified to one volt and then electronically switched to an FM demultiplexing system where appropriate frequency discrimination and signal filtering recover the original analog information
Microwave measurements of the photonic bandgap in a two-dimensional photonic crystal slab
We have measured the photonic bandgap in the transmission of microwaves
through a two-dimensional photonic crystal slab. The structure was constructed
by cementing acrylic rods in a hexagonal closed-packed array to form
rectangular stacks. We find a bandgap centered at approximately 11 GHz, whose
depth, width and center frequency vary with the number of layers in the slab,
angle of incidence and microwave polarization.Comment: 8 pages, 3 figures, submitted to Journal of Applied Physic
Photonic crystal polarizers and polarizing beam splitters
We have experimentally demonstrated polarizers and polarizing beam splitters
based on microwave-scale two-dimensional photonic crystals. Using polarized
microwaves within certain frequency bands, we have observed a squared-sinusoid
(Malus) transmission law when using the photonic crystal as a polarizer. The
photonic crystal also functions as a polarizing beamsplitter; in this
configuration it can be engineered to split incident polarizations in either
order, making it more versatile than conventional, Brewster-angle
beamsplitters.Comment: 7 pages, 3 figures, published Journal Applied Physics 93, 9429 (2003
Adaptive multigrid algorithm for the lattice Wilson-Dirac operator
We present an adaptive multigrid solver for application to the non-Hermitian
Wilson-Dirac system of QCD. The key components leading to the success of our
proposed algorithm are the use of an adaptive projection onto coarse grids that
preserves the near null space of the system matrix together with a simplified
form of the correction based on the so-called gamma_5-Hermitian symmetry of the
Dirac operator. We demonstrate that the algorithm nearly eliminates critical
slowing down in the chiral limit and that it has weak dependence on the lattice
volume
Comparing and contrasting the cognitive effects of hippocampal and ventromedial prefrontal cortex damage: A review of human lesion studies.
The hippocampus and ventromedial prefrontal cortex (vmPFC) are closely connected brain regions whose functions are still debated. In order to offer a fresh perspective on understanding the contributions of these two brain regions to cognition, in this review we considered cognitive tasks that usually elicit deficits in hippocampal-damaged patients (e.g., autobiographical memory retrieval), and examined the performance of vmPFC-lesioned patients on these tasks. We then took cognitive tasks where performance is typically compromised following vmPFC damage (e.g., decision making), and looked at how these are affected by hippocampal lesions. Three salient motifs emerged. First, there are surprising gaps in our knowledge about how hippocampal and vmPFC patients perform on tasks typically associated with the other group. Second, while hippocampal or vmPFC damage seems to adversely affect performance on so-called hippocampal tasks, the performance of hippocampal and vmPFC patients clearly diverges on classic vmPFC tasks. Third, although performance appears analogous on hippocampal tasks, on closer inspection, there are significant disparities between hippocampal and vmPFC patients. Based on these findings, we suggest a tentative hierarchical model to explain the functions of the hippocampus and vmPFC. We propose that the vmPFC initiates the construction of mental scenes by coordinating the curation of relevant elements from neocortical areas, which are then funneled into the hippocampus to build a scene. The vmPFC then engages in iterative re-initiation via feedback loops with neocortex and hippocampus to facilitate the flow and integration of the multiple scenes that comprise the coherent unfolding of an extended mental event
Strong low-frequency quantum correlations from a four-wave mixing amplifier
We show that a simple scheme based on nondegenerate four-wave mixing in a hot
atomic vapor behaves like a near-perfect phase-insensitive optical amplifier,
which can generate bright twin beams with a measured quantum noise reduction in
the intensity difference of more than 8 dB, close to the best optical
parametric amplifiers and oscillators. The absence of a cavity makes the system
immune to external perturbations, and the strong quantum noise reduction is
observed over a large frequency range.Comment: 4 pages, 4 figures. Major rewrite of the previous version. New
experimental results and further analysi
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