2,790 research outputs found
Optical waveguide arrays: quantum effects and PT symmetry breaking
Over the last two decades, advances in fabrication have led to significant
progress in creating patterned heterostructures that support either carriers,
such as electrons or holes, with specific band structure or electromagnetic
waves with a given mode structure and dispersion. In this article, we review
the properties of light in coupled optical waveguides that support specific
energy spectra, with or without the effects of disorder, that are
well-described by a Hermitian tight-binding model. We show that with a
judicious choice of the initial wave packet, this system displays the
characteristics of a quantum particle, including transverse photonic transport
and localization, and that of a classical particle. We extend the analysis to
non-Hermitian, parity and time-reversal () symmetric Hamiltonians
which physically represent waveguide arrays with spatially separated, balanced
absorption or amplification. We show that coupled waveguides are an ideal
candidate to simulate -symmetric Hamiltonians and the transition
from a purely real energy spectrum to a spectrum with complex conjugate
eigenvalues that occurs in them.Comment: 16 pages, 12 figures, Invited Review for European Physics Journal -
Applied Physic
Terahertz Waveguiding in Silicon-Core Fibers
We propose the use of a silicon-core optical fiber for terahertz (THz)
waveguide applications. Finite-difference time-domain simulations have been
performed based on a cylindrical waveguide with a silicon core and silica
cladding. High-resistivity silicon has a flat dispersion over a 0.1 - 3 THz
range, making it viable for propagation of tunable narrowband CW THz and
possibly broadband picosecond pules of THz radiation. Simulations show the
propagation dynamics and the integrated intensity, from which transverse mode
profiles and absorption lengths are extraced. It is found that for 140 - 250
micron core diameters the mode is primarily confined to the core, such that the
overall absorbance is only slightly less than in bulk polycrystalline silicon.Comment: 6 pages, 3 figures, journal submissio
Activity-Dependent mRNA Splicing Controls ER Export and Synaptic Delivery of NMDA Receptors
AbstractActivity-dependent targeting of NMDA receptors (NMDARs) is a key feature of synapse formation and plasticity. Although mechanisms for rapid trafficking of glutamate receptors have been identified, the molecular events underlying chronic accumulation or loss of synaptic NMDARs have remained unclear. Here we demonstrate that activity controls NMDAR synaptic accumulation by regulating forward trafficking at the endoplasmic reticulum (ER). ER export is accelerated by the alternatively spliced C2′ domain of the NR1 subunit and slowed by the C2 splice cassette. This mRNA splicing event at the C2/C2′ site is activity dependent, with C2′ variants predominating upon activity blockade and C2 variants abundant with increased activity. The switch to C2′ accelerates NMDAR forward trafficking by enhancing recruitment of nascent NMDARs to ER exit sites via binding of a divaline motif within C2′ to COPII coats. These results define a novel pathway underlying activity-dependent targeting of glutamate receptors, providing an unexpected mechanistic link between activity, mRNA splicing, and membrane trafficking during excitatory synapse modification
PT-symmetry breaking and maximal chirality in a nonuniform PT-symmetric ring
We study the properties of an N-site tight-binding ring with parity and
time-reversal (PT) symmetric, Hermitian, site-dependent tunneling and a pair of
non-Hermitian, PT-symmetric, loss and gain impurities . The
properties of such lattices with open boundary conditions have been intensely
explored over the past two years. We numerically investigate the PT-symmetric
phase in a ring with a position-dependent tunneling function
that, in an open lattice, leads to a
strengthened PT-symmetric phase, and study the evolution of the PT-symmetric
phase from the open chain to a ring. We show that, generally, periodic boundary
conditions weaken the PT-symmetric phase, although for experimentally relevant
lattice sizes , it remains easily accessible. We show that the
chirality, quantified by the (magnitude of the) average transverse momentum of
a wave packet, shows a maximum at the PT-symmetric threshold. Our results show
that although the wavepacket intensity increases monotonically across the
PT-breaking threshold, the average momentum decays monotonically on both sides
of the threshold.Comment: 11 pages, 5 figures, preprin
Decrease in natural marine hydrocarbon seepage near Coal Oil Point, California, associated with offshore oil production
Prolific natural hydrcarbon seepage occurs offshore of Coal Oil Point in the Santa Barbara Channel, California. Within the water column above submarine vents, plumes of hydrocarbon gas bubbles act as acoustic scattering targets. Using 3.5 kHz sonar data, seep distribution offshore of Coal Oil Point was mapped for August 1996, July 1995, and July 1973. Comparison of the seep distributions over time reveals more than 50% decrease in the areal extent of seepage, accompanied by declines in seep emission volume, in a 13 km2 area above a production oil reservoir. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production are the mechanisms inferred to explain the declines in seep and emission volume
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