89 research outputs found
An initialisation protocol for a CDMA based communications scheme for HFC CATV networks
An initialization protocol for a CDMA based communications system for hybrid fiber coax (HFC) central antenna television (CATV) networks is described. HFC CATV networks are shared medium access networks, employing the coaxial bus principle between an optical network terminator and the subscriber. To realize access in the upstream direction, a multiple access scheme based on CDMA can be used. However, before access is granted in the upstream direction, each cable modem connected to the coaxial bus needs to undergo a startup procedure at activation of the modem to determine timing and physical layer related settings. The initialization mechanism described performs time synchronization of a modem and the determination of the power the modem should use. Next to these physical layer settings, also the identity of a modem is determined during the initialization proces
Spinor Slow-Light and Dirac particles with variable mass
We consider the interaction of two weak probe fields of light with an atomic
ensemble coherently driven by two pairs of standing wave laser fields in a
tripod-type linkage scheme. The system is shown to exhibit a Dirac-like
spectrum for light-matter quasi-particles with multiple dark-states, termed
spinor slow-light polaritons (SSP). They posses an "effective speed of light"
given by the group-velocity of slow-light, and can be made massive by inducing
a small two-photon detuning. Control of the two-photon detuning can be used to
locally vary the mass including a sign flip. This allows e.g. the
implementation of the random-mass Dirac model for which localized zero-energy
(mid-gap) states exist with unsual long-range correlations.Comment: 5 pages, 4 figure
Photonic band-gap properties for two-component slow light
We consider two-component "spinor" slow light in an ensemble of atoms
coherently driven by two pairs of counterpropagating control laser fields in a
double tripod-type linkage scheme. We derive an equation of motion for the
spinor slow light (SSL) representing an effective Dirac equation for a massive
particle with the mass determined by the two-photon detuning. By changing the
detuning the atomic medium acts as a photonic crystal with a controllable band
gap. If the frequency of the incident probe light lies within the band gap, the
light tunnels through the sample. For frequencies outside the band gap, the
transmission probability oscillates with increasing length of the sample. In
both cases the reflection takes place into the complementary mode of the probe
field. We investigate the influence of the finite excited state lifetime on the
transmission and reflection coefficients of the probe light. We discuss
possible experimental implementations of the SSL using alkali atoms such as
Rubidium or Sodium.Comment: 7 figure
Confining stationary light: Dirac dynamics and Klein tunneling
We discuss the properties of 1D stationary pulses of light in atomic ensemble
with electromagnetically induced transparency in the limit of tight spatial
confinement. When the size of the wavepacket becomes comparable or smaller than
the absorption length of the medium, it must be described by a two-component
vector which obeys the one-dimensional two-component Dirac equation with an
effective mass and effective speed of light . Then a fundamental
lower limit to the spatial width in an external potential arises from Klein
tunneling and is given by the effective Compton length . Since and can be externally controlled and can be made small
it is possible to observe effects of the relativistic dispersion for rather low
energies or correspondingly on macroscopic length scales.Comment: 4 pages, 4 figure
Confinement limit of Dirac particles in scalar 1D potentials
We present a general proof that Dirac particles cannot be localized below
their Compton length by symmetric but otherwise arbitrary scalar potentials.
This proof does not invoke the Heisenberg uncertainty relation and thus does
not rely on the nonrelativistic linear momentum relation. Further it is argued
that the result is also applicable for more general potentials, as e.g.
generated by nonlinear interactions. Finally a possible realisation of such a
system is proposed.Comment: 2 page
Dark-State Polaritons for multi-component and stationary light fields
We present a general scheme to determine the loss-free adiabatic
eigensolutions (dark-state polaritons) of the interaction of multiple probe
laser beams with a coherently driven atomic ensemble under conditions of
electromagnetically induced transparency. To this end we generalize the
Morris-Shore transformation to linearized Heisenberg-Langevin equations
describing the coupled light-matter system in the weak excitation limit. For
the simple lambda-type coupling scheme the generalized Morris-Shore
transformation reproduces the dark-state polariton solutions of slow light.
Here we treat a closed-loop dual-V scheme wherein two counter-propagating
control fields generate a quasi stationary pattern of two counter-propagating
probe fields -- so-called stationary light. We show that contrary to previous
predictions,there exists a single unique dark-state polariton; it obeys a
simple propagation equation.Comment: 6 pages, 2 figure
Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal MetalâOrganic Frameworks
Metalâorganic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixedâmetal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixedâmetal metalâorganic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixedâmetal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder Xâray diffraction, Xâray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixedâmetal FeCuBTC material with HKUSTâ1 structure, which contained bimetallic FeâCu paddlewheels as well as monometallic CuâCu and FeâFe units under optimized synthesis conditions. The inâdepth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixedâmetal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixedâmetal metalâorganic framework materials and the thorough characterization thereof is particularly important to derive structureâproperty or structureâactivity correlations
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