303 research outputs found
A saturated red color converter for visible light communication using a blend of star-shaped organic semiconductors
Authors would like to acknowledge the EPSRC for financial support for the UP-VLC (EP/K00042X/1). PJS and IDWS also acknowledge Royal Society Wolfson Research Merit Awards.We report a study of blends of semiconducting polymers as saturated red color converters to replace commercial phosphors in hybrid LEDs for visible light communication (VLC). By blending two star-shaped organic semiconductor molecules, we found a near complete energy transfer (> 90% efficiency) from the green-emitting truxene-cored compound T4BT-B to the red-emitting boron dipyrromethene (BODIPY) cored materials. Furthermore, we have demonstrated the capability of these materials as fast color converters for VLC by measuring their intrinsic optical modulation bandwidth and data rate. The measured 3 dB modulation bandwidth of blends (~55 MHz) is more than 10 times higher than commercially available LED phosphors and also higher than the red-emitting BODIPY color converter alone in solution. The data rate achieved with this blend is 20 times higher than measured with a commercially available phosphor based color converter.PostprintPeer reviewe
Polymer colour converter with very high modulation bandwidth for visible light communications
We thank EPSRC for financial support from the UP-VLC Project Grant (EP/K00042X/1). I.D.W.S. and P.J.S. are Royal Society Wolfson Research Merit Award holders.For white light data communications, broad-band light emitting materials are required, whose emission can be rapidly modulated in intensity. We report the synthesis, photophysics and application of a novel semiconducting polymer for use as a high bandwidth colour converter, to replace commercial phosphors in white LEDs. The high modulation bandwidth (470 MHz) is 140 times higher than that measured using a conventional LED phosphor.Publisher PDFPeer reviewe
Mixed RG Flows and Hydrodynamics at Finite Holographic Screen
We consider quark-gluon plasma with chemical potential and study
renormalization group flows of transport coefficients in the framework of
gauge/gravity duality. We first study them using the flow equations and compare
the results with hydrodynamic results by calculating the Green functions on the
arbitrary slice. Two results match exactly. Transport coefficients at arbitrary
scale is ontained by calculating hydrodynamics Green functions. When either
momentum or charge vanishes, transport coefficients decouple from each other.Comment: 22 pages, 6 figure
Dipole Coupling Effect of Holographic Fermion in the Background of Charged Gauss-Bonnet AdS Black Hole
We investigate the holographic fermions in the charged Gauss-Bonnet
black hole background with the dipole coupling between fermion and gauge field
in the bulk. We show that in addition to the strength of the dipole coupling,
the spacetime dimension and the higher curvature correction in the gravity
background also influence the onset of the Fermi gap and the gap distance. We
find that the higher curvature effect modifies the fermion spectral density and
influences the value of the Fermi momentum for the appearance of the Fermi
surface. There are richer physics in the boundary fermion system due to the
modification in the bulk gravity.Comment: 16 pages, accepted for publication in JHE
Stochastic quantization and holographic Wilsonian renormalization group
We study relation between stochastic quantization and holographic Wilsonian
renormalization group flow. Considering stochastic quantization of the boundary
on-shell actions with the Dirichlet boundary condition for certain bulk
gravity theories, we find that the radial flows of double trace deformations in
the boundary effective actions are completely captured by stochastic time
evolution with identification of the radial coordinate `' with the
stochastic time '' as . More precisely, we investigate Langevin
dynamics and find an exact relation between radial flow of the double trace
couplings and 2-point correlation functions in stochastic quantization. We also
show that the radial evolution of double trace deformations in the boundary
effective action and the stochastic time evolution of the Fokker-Planck action
are the same. We demonstrate this relation with a couple of examples:
(minimally coupled)massless scalar fields in and U(1) vector fields in
.Comment: 1+30 pages, a new subsection is added, references are adde
Equilibrium thermodynamics in modified gravitational theories
We show that it is possible to obtain a picture of equilibrium thermodynamics
on the apparent horizon in the expanding cosmological background for a wide
class of modified gravity theories with the Lagrangian density ,
where is the Ricci scalar and is the kinetic energy of a scalar field
. This comes from a suitable definition of an energy momentum tensor of
the "dark" component that respects to a local energy conservation in the Jordan
frame. In this framework the horizon entropy corresponding to equilibrium
thermodynamics is equal to a quarter of the horizon area in units of
gravitational constant , as in Einstein gravity. For a flat cosmological
background with a decreasing Hubble parameter, globally increases with
time, as it happens for viable inflation and dark energy models. We also
show that the equilibrium description in terms of the horizon entropy is
convenient because it takes into account the contribution of both the horizon
entropy in non-equilibrium thermodynamics and an entropy production
term.Comment: 11 pages, 2 figures, version to appear in Physics Letters B, typos
correcte
Thermal Correlators in Holographic Models with Lifshitz scaling
We study finite temperature effects in two distinct holographic models that
exhibit Lifshitz scaling, looking to identify model independent features in the
dual strong coupling physics. We consider the thermodynamics of black branes
and find different low-temperature behavior of the specific heat. Deformation
away from criticality leads to non-trivial temperature dependence of
correlation functions and we study how the characteristic length scale in the
two point function of scalar operators varies as a function of temperature and
deformation parameters.Comment: 28 pages, 8 figures; typos corrected, references added, published
versio
Maxwell-Chern-Simons Vortices and Holographic Superconductors
We investigate probe limit vortex solutions of a charged scalar field in
Einstein-Maxwell theory in 3+1 dimensions, for an asymptotically AdS
Schwarzschild black hole metric with the addition of an axionic coupling to the
Maxwell field. We show that the inclusion of such a term, together with a
suitable potential for the axion field, can induce an effective Chern-Simons
term on the 2+1 dimensional boundary. We obtain numerical solutions of the
equations of motion and find Maxwell-Chern-Simons like magnetic vortex
configurations, where the magnetic field profile varies with the size of the
effective Chern-Simons coupling. The axion field has a non-trivial profile
inside the AdS bulk but does not condense at spatial infinity.Comment: 17 pages, 5 figures, version accepted for publication in JHE
Bosonic excitations of the AdS4 Reissner-Nordstrom black hole
We study the long-lived modes of the charge density and energy density
correlators in the strongly-coupled, finite density field theory dual to the
AdS4 Reissner-Nordstrom black hole. For small momenta q<<\mu, these correlators
contain a pole due to sound propagation, as well as a pole due to a long-lived,
purely imaginary mode analogous to the \mu=0 hydrodynamic charge diffusion
mode. As the temperature is raised in the range T\lesssim\mu, the sound
attenuation shows no significant temperature dependence. When T\gtrsim\mu, it
quickly approaches the \mu=0 hydrodynamic result where it decreases like 1/T.
It does not share any of the temperature-dependent properties of the 'zero
sound' of Landau Fermi liquids observed in the strongly-coupled D3/D7 field
theory. For such small momenta, the energy density spectral function is
dominated by the sound mode at all temperatures, whereas the charge density
spectral function undergoes a crossover from being dominated by the sound mode
at low temperatures to being dominated by the diffusion mode when T \mu^2/q.
This crossover occurs due to the changing residue at each pole. We also compute
the momentum dependence of these spectral functions and their corresponding
long-lived poles at fixed, low temperatures T<<\mu.Comment: 33 pages, 21 figures, 6 animation
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