744 research outputs found
Graph Sparsification by Edge-Connectivity and Random Spanning Trees
We present new approaches to constructing graph sparsifiers --- weighted
subgraphs for which every cut has the same value as the original graph, up to a
factor of . Our first approach independently samples each
edge with probability inversely proportional to the edge-connectivity
between and . The fact that this approach produces a sparsifier resolves
a question posed by Bencz\'ur and Karger (2002). Concurrent work of Hariharan
and Panigrahi also resolves this question. Our second approach constructs a
sparsifier by forming the union of several uniformly random spanning trees.
Both of our approaches produce sparsifiers with
edges. Our proofs are based on extensions of Karger's contraction algorithm,
which may be of independent interest
Optical Torque from Enhanced Scattering by Multipolar Plasmonic Resonance
We present a theoretical study of the optical angular momentum transfer from
a circularly polarized plane wave to thin metal nanoparticles of different
rotational symmetries. While absorption has been regarded as the predominant
mechanism of torque generation on the nanoscale, we demonstrate numerically how
the contribution from scattering can be enhanced by using multipolar plasmon
resonance. The multipolar modes in non-circular particles can convert the
angular momentum carried by the scattered field, thereby producing
scattering-dominant optical torque, while a circularly symmetric particle
cannot. Our results show that the optical torque induced by resonant scattering
can contribute to 80% of the total optical torque in gold particles. This
scattering-dominant torque generation is extremely mode-specific, and deserves
to be distinguished from the absorption-dominant mechanism. Our findings might
have applications in optical manipulation on the nanoscale as well as new
designs in plasmonics and metamaterials.Comment: main article 20 pages, 4 figures; supplementary material 6 pages, 2
figure
Transformation Optics scheme for two-dimensional materials
Two dimensional optical materials, such as graphene can be characterized by a
surface conductivity. So far, the transformation optics schemes have focused on
three dimensional properties such as permittivity and permeability
. In this paper, we use a scheme for transforming surface currents to
highlight that the surface conductivity transforms in a way different from
and . We use this surface conductivity transformation to
demonstrate an example problem of reducing scattering of plasmon mode from
sharp protrusions in graphene
Electron-photon scattering mediated by localized plasmons: A quantitative analysis by eigen-response theory
We show that the scattering interaction between a high energy electron and a
photon can be strongly enhanced by different types of localized plasmons in a
non-trivial way. The scattering interaction is predicted by an eigen-response
theory, numerically verified by finite-difference-time-domain simulation, and
experimentally verified by cathodoluminescence spectroscopy. We find that the
scattering interaction associated with dark plasmons can be as strong as that
of bright plasmons. Such a strong interaction may offer new opportunities to
improve single-plasmon detection and high-resolution characterization
techniques for high quality plasmonic materials.Comment: 4 pages, 4 figures (excluding Supporting Information
Photon Emission Rate Engineering using Graphene Nanodisc Cavities
In this work, we present a systematic study of the plasmon modes in a system
of vertically stacked pair of graphene discs. Quasistatic approximation is used
to model the eigenmodes of the system. Eigen-response theory is employed to
explain the spatial dependence of the coupling between the plasmon modes and a
quantum emitter. These results show a good match between the semi-analytical
calculation and full-wave simulations. Secondly, we have shown that it is
possible to engineer the decay rates of a quantum emitter placed inside and
near this cavity, using Fermi level tuning, via gate voltages and variation of
emitter location and polarization. We highlighted that by coupling to the
bright plasmon mode, the radiative efficiency of the emitter can be enhanced
compared to the single graphene disc case, whereas the dark plasmon mode
suppresses the radiative efficiency
Tunable light-matter interaction and the role of hyperbolicity in graphene-hBN system
Hexagonal boron nitride (hBN) is a natural hyperbolic material which can also
accommodate highly dispersive surface phonon-polariton modes. In this paper, we
examine theoretically the mid-infrared optical properties of graphene-hBN
heterostructures derived from their coupled plasmon-phonon modes. We found that
the graphene plasmon couples differently with the phonons of the two
Reststrahlen bands, owing to their different hyperbolicity. This also leads to
distinctively different interaction between an external quantum emitter and the
plasmon-phonon modes in the two bands, leading to substantial modification of
its spectrum. The coupling to graphene plasmons allows for additional gate
tunability in the Purcell factor, and narrow dips in its emission spectra
Excitation and Imaging of Resonant Optical Modes of Au Triangular Nano-Antennas Using Cathodoluminescence Spectroscopy
Cathodoluminescence (CL) imaging spectroscopy is an important technique to
understand resonant behavior of optical nanoantennas. We report high-resolution
CL spectroscopy of triangular gold nanoantennas designed with near-vacuum
effective index and very small metal-substrate interface. This design helped in
addressing issues related to background luminescence and shifting of dipole
modes beyond visible spectrum. Spatial and spectral investigations of various
plasmonic modes are reported. Out-of-plane dipole modes excited with vertically
illuminated electron beam showed high-contrast tip illumination in panchromatic
imaging. By tilting the nanostructures during fabrication, in-plane dipole
modes of antennas were excited. Finite-difference time-domain simulations for
electron and optical excitations of different modes showed excellent agreement
with experimental results. Our approach of efficiently exciting antenna modes
by using low index substrates is confirmed both with experiments and numerical
simulations. This should provide further insights into better understanding of
optical antennas for various applications.Comment: To be published in JVST B (accepted, Sep 2010) (15 pages, 6 figures,
originally presented at EIPBN 2010
Chiral plasmon in gapped Dirac systems
We study the electromagnetic response and surface electromagnetic modes in a
generic gapped Dirac material under pumping with circularly polarized light.
The valley imbalance due to pumping leads to a net Berry curvature, giving rise
to a finite transverse conductivity. We discuss the appearance of nonreciprocal
chiral edge modes, their hybridization and waveguiding in a nanoribbon
geometry, and giant polarization rotation in nanoribbon arrays
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