104 research outputs found
Electrically driven photon emission from individual atomic defects in monolayer WS2.
Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources
Resonant and bound states of charged defects in two-dimensional semiconductors
A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS2 using a combination of scanning tunneling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multivalley valence band structure of WS2, whereby localized states originating from the secondary valence band maximum at Γ hybridize with continuum states from the primary valence band maximum at K/K′. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds
Superlinear emission in bare perovskite: amplified spontaneous emission in disordered film versus single crystal lasing
Abstract We present an experimental study concerning the superlinear emission in organic-inorganic halide perovskites. Microphotoluminescence experiments under CW and picosecond excitation condition at low temperature and near field optical photoluminescence spectra at room temperature provide clear evidence of the very different origin of the superlinear regime in disordered films and microplates/microwires. Insights on the origin of modal structures of the emission spectra in the high excitation regime will be given by polarization-resolved photoluminescence experiments
Fabrication of Nanostructured GaAs/AlGaAs Waveguide for Low-Density Polariton Condensation from a Bound State in the Continuum
Exciton-polaritons are hybrid light-matter states that arise from strong
coupling between an exciton resonance and a photonic cavity mode. As bosonic
excitations, they can undergo a phase transition to a condensed state that can
emit coherent light without a population inversion. This aspect makes them good
candidates for thresholdless lasers, yet short exciton-polariton lifetime has
made it difficult to achieve condensation at very low power densities. In this
sense, long-lived symmetry-protected states are excellent candidates to
overcome the limitations that arise from the finite mirror reflectivity of
monolithic microcavities. In this work we use a photonic symmetry protected
bound state in the continuum coupled to an excitonic resonance to achieve
state-of-the-art polariton condensation threshold in GaAs/AlGaAs waveguide.
Most important, we show the influence of fabrication control and how surface
passivation via atomic layer deposition provides a way to reduce exciton
quenching at the grating sidewalls
Label-free in situ imaging of lignification in the cell wall of low lignin transgenic Populus trichocarpa
Chemical imaging by confocal Raman microscopy has been used for the visualization of the cellulose and lignin distribution in wood cell walls. Lignin reduction in wood can be achieved by, for example, transgenic suppression of a monolignol biosynthesis gene encoding 4-coumarate-CoA ligase (4CL). Here, we use confocal Raman microscopy to compare lignification in wild type and lignin-reduced 4CL transgenic Populus trichocarpa stem wood with spatial resolution that is sub-μm. Analyzing the lignin Raman bands in the spectral region between 1,600 and 1,700 cm−1, differences in lignin signal intensity and localization are mapped in situ. Transgenic reduction of lignin is particularly pronounced in the S2 wall layer of fibers, suggesting that such transgenic approach may help overcome cell wall recalcitrance to wood saccharification. Spatial heterogeneity in the lignin composition, in particular with regard to ethylenic residues, is observed in both samples
Nanoantennas for visible and infrared radiation
Nanoantennas for visible and infrared radiation can strongly enhance the
interaction of light with nanoscale matter by their ability to efficiently link
propagating and spatially localized optical fields. This ability unlocks an
enormous potential for applications ranging from nanoscale optical microscopy
and spectroscopy over solar energy conversion, integrated optical
nanocircuitry, opto-electronics and density-ofstates engineering to
ultra-sensing as well as enhancement of optical nonlinearities. Here we review
the current understanding of optical antennas based on the background of both
well-developed radiowave antenna engineering and the emerging field of
plasmonics. In particular, we address the plasmonic behavior that emerges due
to the very high optical frequencies involved and the limitations in the choice
of antenna materials and geometrical parameters imposed by nanofabrication.
Finally, we give a brief account of the current status of the field and the
major established and emerging lines of investigation in this vivid area of
research.Comment: Review article with 76 pages, 21 figure
Direct visualization of the charge transfer in Graphene/-RuCl heterostructure
We investigate the electronic properties of a graphene and -ruthenium
trichloride (hereafter RuCl) heterostructure, using a combination of
experimental and theoretical techniques. RuCl is a Mott insulator and a
Kitaev material, and its combination with graphene has gained increasing
attention due to its potential applicability in novel electronic and
optoelectronic devices. By using a combination of spatially resolved
photoemission spectroscopy, low energy electron microscopy, and density
functional theory (DFT) calculations we are able to provide a first direct
visualization of the massive charge transfer from graphene to RuCl, which
can modify the electronic properties of both materials, leading to novel
electronic phenomena at their interface. The electronic band structure is
compared to DFT calculations that confirm the occurrence of a Mott transition
for RuCl. Finally, a measurement of spatially resolved work function allows
for a direct estimate of the interface dipole between graphene and RuCl.
The strong coupling between graphene and RuCl could lead to new ways of
manipulating electronic properties of two-dimensional lateral heterojunction.
Understanding the electronic properties of this structure is pivotal for
designing next generation low-power opto-electronics devices
Mapping Local Charge Recombination Heterogeneity by Multidimensional Nanospectroscopic Imaging
As materials functionality becomes more dependent on local physical and electronic properties,
the importance of optically probing matter with true nanoscale spatial resolution has increased.
In this work, we mapped the influence of local trap states within individual nanowires on carrier
recombination with deeply subwavelength resolution. This is achieved using multidimensional
nanospectroscopic imaging based on a nano-optical device. Placed at the end of a scan probe,
the device delivers optimal near-field properties, including highly efficient far-field to near-field
coupling, ultralarge field enhancement, nearly background-free imaging, independence from
sample requirements, and broadband operation. We performed ~40-nanometer–resolution
hyperspectral imaging of indium phosphide nanowires via excitation and collection through
the probes, revealing optoelectronic structure along individual nanowires that is not accessible
with other methods
Autonomous Investigations over WS and Au{111} with Scanning Probe Microscopy
Individual atomic defects in 2D materials impact their macroscopic
functionality. Correlating the interplay is challenging, however, intelligent
hyperspectral scanning tunneling spectroscopy (STS) mapping provides a feasible
solution to this technically difficult and time consuming problem. Here, dense
spectroscopic volume is collected autonomously via Gaussian process regression,
where convolutional neural networks are used in tandem for spectral
identification. Acquired data enable defect segmentation, and a workflow is
provided for machine-driven decision making during experimentation with
capability for user customization. We provide a means towards autonomous
experimentation for the benefit of both enhanced reproducibility and
user-accessibility. Hyperspectral investigations on WS sulfur vacancy sites
are explored, which is combined with local density of states confirmation on
the Au{111} herringbone reconstruction. Chalcogen vacancies, pristine WS,
Au face-centered cubic, and Au hexagonal close packed regions are examined and
detected by machine learning methods to demonstrate the potential of artificial
intelligence for hyperspectral STS mapping.Comment: Updates from final journal publicatio
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