221 research outputs found
Transverse electric plasmons in bilayer graphene
We predict the existence of transverse electric (TE) plasmons in bilayer
graphene. We find that their plasmonic properties are much more pronounced in
bilayer than in monolayer graphene, in a sense that they can get more localized
at frequencies just below ~eV for adequate doping values. This
is a consequence of the perfectly nested bands in bilayer graphene which are
separated by ~eV
Binary matrices of optimal autocorrelations as alignment marks
We define a new class of binary matrices by maximizing the peak-sidelobe
distances in the aperiodic autocorrelations. These matrices can be used as
robust position marks for in-plane spatial alignment. The optimal square
matrices of dimensions up to 7 by 7 and optimal diagonally-symmetric matrices
of 8 by 8 and 9 by 9 were found by exhaustive searches.Comment: 8 pages, 6 figures and 1 tabl
Non-Abelian Generalizations of the Hofstadter model: Spin-orbit-coupled Butterfly Pairs
The Hofstadter model, well-known for its fractal butterfly spectrum,
describes two-dimensional electrons under a perpendicular magnetic field, which
gives rise to the integer quantum hall effect. Inspired by the real-space
building blocks of non-Abelian gauge fields from a recent experiment [Science,
365, 1021 (2019)], we introduce and theoretically study two non-Abelian
generalizations of the Hofstadter model. Each model describes two pairs of
Hofstadter butterflies that are spin-orbit coupled. In contrast to the original
Hofstadter model that can be equivalently studied in the Landau and symmetric
gauges, the corresponding non-Abelian generalizations exhibit distinct spectra
due to the non-commutativity of the gauge fields. We derive the genuine
(necessary and sufficient) non-Abelian condition for the two models from the
commutativity of their arbitrary loop operators. At zero energy, the models are
gapless and host Weyl and Dirac points protected by internal and crystalline
symmetries. Double (8-fold), triple (12-fold), and quadrupole (16-fold) Dirac
points also emerge, especially under equal hopping phases of the non-Abelian
potentials. At other fillings, the gapped phases of the models give rise to
topological insulators. We conclude by discussing possible
schemes for the experimental realizations of the models in photonic platforms
Weyl points and line nodes in gapless gyroid photonic crystals
Weyl points and line nodes are three-dimensional linear point- and
line-degeneracies between two bands. In contrast to Dirac points, which are
their two-dimensional analogues, Weyl points are stable in the momentum space
and the associated surface states are predicted to be topologically
non-trivial. However, Weyl points are yet to be discovered in nature. Here, we
report photonic crystals, based on the double-gyroid structures, exhibiting
frequency-isolated Weyl points with intricate phase diagrams. The surface
states associated with the non-zero Chern numbers are demonstrated. Line nodes
are also found in similar geometries; the associated surface states are shown
to be flat bands. Our results are readily experimentally realizable at both
microwave and optical frequencies.Comment: 6 figures and 8 pages including the supplementary informatio
Plasmonics in graphene at infra-red frequencies
We point out that plasmons in doped graphene simultaneously enable low-losses
and significant wave localization for frequencies below that of the optical
phonon branch eV. Large plasmon losses occur in
the interband regime (via excitation of electron-hole pairs), which can be
pushed towards higher frequencies for higher doping values. For sufficiently
large dopings, there is a bandwidth of frequencies from up to
the interband threshold, where a plasmon decay channel via emission of an
optical phonon together with an electron-hole pair is nonegligible. The
calculation of losses is performed within the framework of a random-phase
approximation and number conserving relaxation-time approximation. The measured
DC relaxation-time serves as an input parameter characterizing collisions with
impurities, whereas the contribution from optical phonons is estimated from the
influence of the electron-phonon coupling on the optical conductivity. Optical
properties of plasmons in graphene are in many relevant aspects similar to
optical properties of surface plasmons propagating on dielectric-metal
interface, which have been drawing a lot of interest lately because of their
importance for nanophotonics. Therefore, the fact that plasmons in graphene
could have low losses for certain frequencies makes them potentially
interesting for nanophotonic applications.Comment: 5 figure
Large-Scale Optical Neural Networks based on Photoelectric Multiplication
Recent success in deep neural networks has generated strong interest in
hardware accelerators to improve speed and energy consumption. This paper
presents a new type of photonic accelerator based on coherent detection that is
scalable to large () networks and can be operated at high (GHz)
speeds and very low (sub-aJ) energies per multiply-and-accumulate (MAC), using
the massive spatial multiplexing enabled by standard free-space optical
components. In contrast to previous approaches, both weights and inputs are
optically encoded so that the network can be reprogrammed and trained on the
fly. Simulations of the network using models for digit- and
image-classification reveal a "standard quantum limit" for optical neural
networks, set by photodetector shot noise. This bound, which can be as low as
50 zJ/MAC, suggests performance below the thermodynamic (Landauer) limit for
digital irreversible computation is theoretically possible in this device. The
proposed accelerator can implement both fully-connected and convolutional
networks. We also present a scheme for back-propagation and training that can
be performed in the same hardware. This architecture will enable a new class of
ultra-low-energy processors for deep learning.Comment: Text: 10 pages, 5 figures, 1 table. Supplementary: 8 pages, 5,
figures, 2 table
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