324 research outputs found
Optical spin pumping induced pseudo-magnetic field in two dimensional heterostructures
Two dimensional heterostructures are likely to provide new avenues for the
manipulation of magnetization that is crucial for spintronics or
magnetoelectronics. Here, we demonstrate that optical spin pumping can generate
a large effective magnetic field in two dimensional MoSe2/WSe2
heterostructures. We determine the strength of the generated field by
polarization-resolved measurement of the interlayer exciton photoluminescence
spectrum: the measured splitting exceeding 10 milli-electron volts (meV)
between the emission originating from the two valleys corresponds to an
effective magnetic field of ~ 30 T. The strength of this optically induced
field can be controlled by the excitation light polarization. Our finding opens
up new possibilities for optically controlled spintronic devices based on van
der Waals heterostructures
Stacking sequence determines Raman intensities of observed interlayer shear modes in 2D layered materials - A general bond polarizability model
2D layered materials have recently attracted tremendous interest due to their
fascinating properties and potential applications. The interlayer interactions
are much weaker than the intralayer bonds, allowing the as-synthesized
materials to exhibit different stacking sequences (e.g. ABAB, ABCABC), leading
to different physical properties. Here, we show that regardless of the space
group of the 2D material, the Raman frequencies of the interlayer shear modes
observed under the typical configuration blue shift for AB stacked materials,
and red shift for ABC stacked materials, as the number of layers increases. Our
predictions are made using an intuitive bond polarizability model which shows
that stacking sequence plays a key role in determining which interlayer shear
modes lead to the largest change in polarizability (Raman intensity); the modes
with the largest Raman intensity determining the frequency trends. We present
direct evidence for these conclusions by studying the Raman modes in few layer
graphene, MoS2, MoSe2, WSe2 and Bi2Se3, using both first principles
calculations and Raman spectroscopy. This study sheds light on the influence of
stacking sequence on the Raman intensities of intrinsic interlayer modes in 2D
layered materials in general, and leads to a practical way of identifying the
stacking sequence in these materials.Comment: 30 pages, 8 figure
Top-Down Structure and Device Fabrication using \u3ci\u3eIn Situ\u3c/i\u3e Nanomachining
We demonstrate the potential of an alternative tool for the fabrication of nanoscale structures and devices. A nanoindenter integrated with an atomic force microscope is shown to be a powerful machine tool for cutting precise length nanowires or nanobelts and for manipulating the shortened wires. We also demonstrate its utility in cutting grooves and fabricating dents (or periodic arrays of dents) in ZnSnanobelts. This approach permits the direct mechanical machining of nanodevices that are supported on a substrate without the inherent complications of e beam or photolithography
Observation of forbidden phonons and dark excitons by resonance Raman scattering in few-layer WS
The optical properties of the two-dimensional (2D) crystals are dominated by
tightly bound electron-hole pairs (excitons) and lattice vibration modes
(phonons). The exciton-phonon interaction is fundamentally important to
understand the optical properties of 2D materials and thus help develop
emerging 2D crystal based optoelectronic devices. Here, we presented the
excitonic resonant Raman scattering (RRS) spectra of few-layer WS excited
by 11 lasers lines covered all of A, B and C exciton transition energies at
different sample temperatures from 4 to 300 K. As a result, we are not only
able to probe the forbidden phonon modes unobserved in ordinary Raman
scattering, but also can determine the bright and dark state fine structures of
1s A exciton. In particular, we also observed the quantum interference between
low-energy discrete phonon and exciton continuum under resonant excitation. Our
works pave a way to understand the exciton-phonon coupling and many-body
effects in 2D materials.Comment: 14 pages, 11 figure
Chiral plasmonics and enhanced chiral light-matter interactions
International audienceChirality, which describes the broken mirror symmetry in geometric structures, exists macroscopically in our daily life as well as microscopically down to molecular levels. Correspondingly, chiral molecules interact differently with circularly polarized light exhibiting opposite handedness (left-handed and right-handed). However, the interaction between chiral molecules and chiral light is very weak. In contrast, artificial chiral plasmonic structures can generate “super-chiral” plasmonic near-field, leading to enhanced chiral light-matter (or chiroptical) interactions. The “super-chiral” near-field presents different amplitude and phase under opposite handedness incidence, which can be utilized to engineer linear and nonlinear chiroptical interactions. Specifically, in the interaction between quantum emitters and chiral plasmonic structures, the chiral hot spots can favour the emission with a specific handedness. This article reviews the state-of-the-art research on the design, fabrication and chiroptical response of different chiral plasmonic nanostructures or metasurfaces. This review also discusses enhanced chiral light-matter interactions that are essential for applications like chirality sensing, chiral selective light emitting and harvesting. In the final part, the review ends with a perspective on future directions of chiral plasmonics
Anomalous Frequency Trends in MoS2 Thin Films Attributed to Surface Effects
The layered dichalcogenide MoS2 has many unique physical properties in low
dimensions. Recent experimental Raman spectroscopies report an anomalous blue
shift of the in-plane E2g1 mode with decreasing thickness, a trend that is not
understood. Here, we combine experimental Raman scattering and theoretical
studies to clarify and explain this trend. Special attention is given to
understanding the surface effect on Raman frequencies by using a force
constants model based on first-principles calculations. Surface effects refer
to the larger Mo-S force constants at the surface of thin film MoS2, which
results from a loss of neighbours in adjacent MoS2 layers. Without surface
effects, the frequencies of both out-of-plane A1g and in-plane E2g1 modes
decrease with decreasing thickness. However, the E2g1 mode blue shifts while
the A1g mode red shifts once the surface effect is included, in agreement with
the experiment. Our results show that competition between the thickness effect
and the surface effect determines the mechanical properties of two-dimensional
MoS2, which we believe applies to other layered materials
Learning Multimodal Volumetric Features for Large-Scale Neuron Tracing
The current neuron reconstruction pipeline for electron microscopy (EM) data
usually includes automatic image segmentation followed by extensive human
expert proofreading. In this work, we aim to reduce human workload by
predicting connectivity between over-segmented neuron pieces, taking both
microscopy image and 3D morphology features into account, similar to human
proofreading workflow. To this end, we first construct a dataset, named
FlyTracing, that contains millions of pairwise connections of segments
expanding the whole fly brain, which is three orders of magnitude larger than
existing datasets for neuron segment connection. To learn sophisticated
biological imaging features from the connectivity annotations, we propose a
novel connectivity-aware contrastive learning method to generate dense
volumetric EM image embedding. The learned embeddings can be easily
incorporated with any point or voxel-based morphological representations for
automatic neuron tracing. Extensive comparisons of different combination
schemes of image and morphological representation in identifying split errors
across the whole fly brain demonstrate the superiority of the proposed
approach, especially for the locations that contain severe imaging artifacts,
such as section missing and misalignment. The dataset and code are available at
https://github.com/Levishery/Flywire-Neuron-Tracing.Comment: 9 pages, 6 figures, AAAI 2024 accepte
Topological Single Photon Emission from Quantum Emitter Chains
We develop a scheme of generating highly indistinguishable single photons
from an active quantum Su-Schrieffer-Heeger chain made from a collection of
noisy quantum emitters. Surprisingly, the single photon emission spectrum of
the active quantum chain is extremely narrow compared to that of a single
emitter or topologically trivial chain. Moreover, this effect becomes
dramatically strong close to the non-trivial-to-trivial phase transition point.
Using this effect, we show that the single photon linewidth of a long
topological quantum chain can become arbitrarily narrow, constituting an ideal
source of indistinguishable single photons. Finally, taking specific examples
of actual quantum emitters, we provide a microscopic and quantitative analysis
of our model and analyze the most important parameters in view of the
experimental realization
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