569 research outputs found
Magneto-electric point scattering theory for metamaterial scatterers
We present a new, fully analytical point scattering model which can be
applied to arbitrary anisotropic magneto-electric dipole scatterers, including
split ring resonators (SRRs), chiral and anisotropic plasmonic scatterers. We
have taken proper account of reciprocity and radiation damping for electric and
magnetic scatterers with any general polarizability tensor. Specifically, we
show how reciprocity and energy balance puts constraints on the electrodynamic
responses arbitrary scatterers can have to light. Our theory sheds new light on
the magnitude of cross sections for scattering and extinction, and for instance
on the emergence of structural chirality in the optical response of
geometrically non-chiral scatterers like SRRs. We apply the model to SRRs and
discuss how to extract individual components of the polarizability matrix and
extinction cross sections. Finally, we show that our model describes well the
extinction of stereo-dimers of split rings, while providing new insights in the
underlying coupling mechanisms.Comment: 12 pages, 3 figure
Attosecond streaking in a nano-plasmonic field
A theoretical study of the application of attosecond streaking spectroscopy to
time-resolved studies of the plasmonic fields surrounding isolated, resonantly
excited spherical nanoparticles is presented. A classification of the
different regimes in attosecond streaking is proposed and identified in our
results that are derived from Mie calculations of plasmon fields, coupled to
classical electron trajectory simulations. It is shown that in an attosecond
streaking experiment, the electrons are almost exclusively sensitive to the
component of the field parallel to the direction in which they are detected.
This allows one to probe the different components of the field individually by
resolving the angle of emission of the electrons. Finally, simulations based
on fields calculated by finite-difference time-domain (FDTD) are compared with
the results obtained using Mie fields. The two are found to be in good
agreement with each other, supporting the notion that FDTD methods can be used
to reliably investigate non-spherical structures
Active biopolymer networks generate scale-free but euclidean clusters
We report analytical and numerical modelling of active elastic networks,
motivated by experiments on crosslinked actin networks contracted by myosin
motors. Within a broad range of parameters, the motor-driven collapse of active
elastic networks leads to a critical state. We show that this state is
qualitatively different from that of the random percolation model.
Intriguingly, it possesses both euclidean and scale-free structure with Fisher
exponent smaller than . Remarkably, an indistinguishable Fisher exponent and
the same euclidean structure is obtained at the critical point of the random
percolation model after absorbing all enclaves into their surrounding clusters.
We propose that in the experiment the enclaves are absorbed due to steric
interactions of network elements. We model the network collapse, taking into
account the steric interactions. The model shows how the system robustly drives
itself towards the critical point of the random percolation model with absorbed
enclaves, in agreement with the experiment.Comment: 6 pages, 7 figure
Near-field imaging and frequency tuning of a high-Q photonic crystal membrane microcavity
We discuss experimental studies of the interaction between a nanoscopic
object and a photonic crystal membrane resonator of quality factor Q=55000. By
controlled actuation of a glass fiber tip in the near-field of a photonic
crystal, we constructed a complete spatio-spectral map of the resonator mode
and its coupling with the fiber-tip. On the one hand, our findings demonstrate
that scanning probes can profoundly influence the optical characteristics and
the near-field images of photonic devices. On the other hand, we show that the
introduction of a nanoscopic object provides a low-loss method for on-command
tuning of a photonic crystal resonator frequency. Our results are in a very
good agreement with the predictions of a combined numerical/analytical theory.Comment: 9 pages, 4 figure
Local density of states, spectrum, and far-field interference of surface plasmon polaritons probed by cathodoluminescence
The surface plasmon polariton (SPP) field intensity in the vicinity of gratings patterned in an otherwise planar gold surface is spatially resolved using cathodoluminescence (CL). A detailed theoretical analysis is presented that successfully explains the measured CL signal based upon interference of transition radiation directly generated by electron impact and SPPs launched by the electron and outcoupled by the grating. The measured spectral dependence of the SPP yield per incoming electron is in excellent agreement with rigorous electromagnetic calculations. The CL emission is shown to be similar to that of a dipole oriented perpendicular to the surface and situated at the point of electron impact, which allows us to establish a solid connection between the CL signal and the photonic local density of states associated to the SPPs
Backaction in metasurface etalons
We consider the response of etalons created by a combination of a
conventional mirror and a metasurface, composed of a periodic lattice of metal
scatterers with a resonant response. This geometry has been used previously for
perfect absorption, in so-called Salisbury screens, and for hybridization of
localized plasmons with Fabry-Perot resonances. The particular aspect we
address is if one can assume an environment-independent reflectivity for the
metasurface when calculating the reflectivity of the composite system, as in a
standard Fabry-Perot analysis, or whether the fact that the metasurface
interacts with its own mirror image renormalizes its response. Using lattice
sum theory, we take into account all possible retarded dipole-dipole
interactions of scatterers in the metasurface amongst each other, and through
the mirror. We show that while a layer-by-layer Fabry-Perot formalism captures
the main qualitative features of metasurface etalons, in fact the mirror
modifies both the polarizability and reflectivity of the metasurface in a
fashion that is akin to Drexhage's modification of the radiative properties of
a single dipole.Comment: 10 pages, 5 figure
Multi-scale strain-stiffening of semiflexible bundle networks
Bundles of polymer filaments are responsible for the rich and unique
mechanical behaviors of many biomaterials, including cells and extracellular
matrices. In fibrin biopolymers, whose nonlinear elastic properties are crucial
for normal blood clotting, protofibrils self-assemble and bundle to form
networks of semiflexible fibers. Here we show that the extraordinary
strain-stiffening response of fibrin networks is a direct reflection of the
hierarchical architecture of the fibrin fibers. We measure the rheology of
networks of unbundled protofibrils and find excellent agreement with an affine
model of extensible wormlike polymers. By direct comparison with these data, we
show that physiological fibrin networks composed of thick fibers can be modeled
as networks of tight protofibril bundles. We demonstrate that the tightness of
coupling between protofibrils in the fibers can be tuned by the degree of
enzymatic intermolecular crosslinking by the coagulation Factor XIII.
Furthermore, at high stress, the protofibrils contribute independently to the
network elasticity, which may reflect a decoupling of the tight bundle
structure. The hierarchical architecture of fibrin fibers can thus account for
the nonlinearity and enormous elastic resilience characteristic of blood clots.Comment: 27 pages including 8 figures and Supplementary Dat
From weak to strong coupling of localized surface plasmons to guided modes in a luminescent slab
We investigate a periodic array of aluminum nanoantennas embedded in a
light-emitting slab waveguide. By varying the waveguide thickness we
demonstrate the transition from weak to strong coupling between localized
surface plasmons in the nanoantennas and refractive index guided modes in the
waveguide. We experimentally observe a non-trivial relationship between
extinction and emission dispersion diagrams across the weak to strong coupling
transition. These results have implications for a broad class of photonic
structures where sources are embedded within coupled resonators. For
nanoantenna arrays, strong vs. weak coupling leads to drastic modifications of
radiation patterns without modifying the nanoantennas themselves, thereby
representing an unprecedented design strategy for nanoscale light sources
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