161 research outputs found
Bulk photonic metamaterial with hyperbolic dispersion
In this work, we demonstrate a self-standing bulk three-dimensional
metamaterial based on the network of silver nanowires in an alumina membrane.
This constitutes an anisotropic effective medium with hyperbolic dispersion,
which can be used in sub-diffraction imaging or optical cloaks. Highly
anisotropic dielectric constants of the material range from positive to
negative, and the transmitted laser beam shifts both toward the normal to the
surface, as in regular dielectrics, and off the normal, as in anisotropic
dielectrics with the refraction index smaller than one. The designed photonic
metamaterial is the thickest reported in the literature, both in terms of its
physical size 1cm x 1cm x 51 mm, and the number of vacuum wavelengths, N=61 at
l=0.84 mm.Comment: 6 pages, 4 figur
Light intensity-induced phase transitions in graphene oxide doped polyvinylidene fluoride
The coupling of light with low-frequency functionalities of dielectrics and liquid crystals and an ability to turn “on” and “off” the pyro-, piezo-, or ferro- electric properties of materials on demand by optical means leads to fascinating science and device applications. Moreover, to achieve all-optical control in nano-circuits, the coupling of the light with mechanical degrees of freedom is highly desirable and has been elusive until recently. In this work, we report on the light intensity-induced structural phase transitions in graphene oxide doped piezoelectric polyvinylidene fluoride (PVDF) film observed by micro-Raman spectroscopy. Increasing the laser power results in a steady transformation of the Raman spectrum featured piezoelectric phase to one of non-piezoelectric structure. This effect is accompanied by volumetric change of a PVDF unit cell by a factor of two, useful for a photostriction materials application. Furthermore, we observed the reversible switching of α and phases as a function of the light intensity (laser power between 5.7–31.3 mW). This opens up a new route for multi-functionality control where strain, piezoelectric constants and polarization can be modified by light
Optical Properties of ZnP2 Nanoparticles in Zeolite
We report that for the first time the nanoparticles of II-V semiconductor
(ZnP2) were prepared and studied. ZnP2 nanoparticles were prepared by
incorporation into zeolite Na-X matrix. Absorption, diffuse reflection (DR) and
photoluminescence (PL) spectra of the ZnP2 nanoclusters incorporated into the
supercages of zeolite Na-X were measured at the temperature 77 K. Five bands
B1-B5 are observed in both the DR and PL spectra demonstrating the blue shift
from the line of free exciton in bulk crystal. We attribute the B1-B5 bands to
some stable nanoclusters with size less than the size of zeolite Na-X
supercage. We observed Stokes shift of the PL bands from the respective
absorption bands. The nonmonotonic character of its dependence on the cluster
size can be explained as the result of competition of the Frank-Condon shift
and the shift due to electronic relaxation.Comment: Submitted to Microporous and Mesoporous Material
Magneto-optical spectra of closely spaced magnetite nanoparticles
The Faraday rotation spectrum of composites containing magnetite nanoparticles is found to be dependent on the interparticle spacing of the constituent nanoparticles. The composite materials are prepared by combining chemically synthesized Fe
3O4 smagnetited nanoparticles s8-nm diameterd and polysmethylmethacrylated . Composites are made containing a range of nanoparticle concentrations. The peak of the main spectral feature depends on nanoparticle concentration; this peak is observed to shift from approximately 470 nm for sdilute compositesd to 540 nm concentrated . We present a theory based on the discrete-dipole approximation which accounts for
optical coupling between magnetite particles. Qualitative correlations between theoretical calculations and experimental data suggest that the shifts in spectral peak position depend on both interparticle distance and geometrical configuratio
Perfect and near perfect adaptation in a model of bacterial chemotaxis
The signaling apparatus mediating bacterial chemotaxis can adapt to a wide
range of persistent external stimuli. In many cases, the bacterial activity
returns to its pre-stimulus level exactly and this "perfect adaptability" is
robust against variations in various chemotaxis protein concentrations. We
model the bacterial chemotaxis signaling pathway, from ligand binding to CheY
phosphorylation. By solving the steady-state equations of the model
analytically, we derive a full set of conditions for the system to achieve
perfect adaptation. The conditions related to the phosphorylation part of the
pathway are discovered for the first time, while other conditions are
generalization of the ones found in previous works. Sensitivity of the perfect
adaptation is evaluated by perturbing these conditions. We find that, even in
the absence of some of the perfect adaptation conditions, adaptation can be
achieved with near perfect precision as a result of the separation of scales in
both chemotaxis protein concentrations and reaction rates, or specific
properties of the receptor distribution in different methylation states. Since
near perfect adaptation can be found in much larger regions of the parameter
space than that defined by the perfect adaptation conditions, their existence
is essential to understand robustness in bacterial chemotaxis.Comment: 16 pages, 9 figure
Dynamic receptor team formation can explain the high signal transduction gain in E. coli
Evolution has provided many organisms with sophisticated sensory systems that
enable them to respond to signals in their environment. The response frequently
involves alteration in the pattern of movement, such as the chemokinesis of the
bacterium Escherichia coli, which swims by rotating its flagella. When rotated
counterclockwise (CCW) the flagella coalesce into a propulsive bundle,
producing a relatively straight ``run'', and when rotated clockwise (CW) they
fly apart, resulting in a ``tumble'' which reorients the cell with little
translocation. A stochastic process generates the runs and tumbles, and in a
chemoeffector gradient runs that carry the cell in a favorable direction are
extended. The overall structure of the signal transduction pathways is
well-characterized in E. coli, but important details are still not understood.
Only recently has a source of gain in the signal transduction network been
identified experimentally, and here we present a mathematical model based on
dynamic assembly of receptor teams that can explain this observation.Comment: Accepted for publication in the Biophysical Journa
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