121 research outputs found
Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source
We report on the development of an ultrafast Transmission Electron Microscope
based on a cold field emission source which can operate in either DC or
ultrafast mode. Electron emission from a tungsten nanotip is triggered by
femtosecond laser pulses which are tightly focused by optical components
integrated inside a cold field emission source close to the cathode. The
properties of the electron probe (brightness, angular current density,
stability) are quantitatively determined. The measured brightness is the
largest reported so far for UTEMs. Examples of imaging, diffraction and
spectroscopy using ultrashort electron pulses are given. Finally, the potential
of this instrument is illustrated by performing electron holography in the
off-axis configuration using ultrashort electron pulses.Comment: 23 pages, 9 figure
Bridging nano-optics and condensed matter formalisms in a unified description of inelastic scattering of relativistic electron beams
In the last decades, the blossoming of experimental breakthroughs in the
domain of electron energy loss spectroscopy (EELS) has triggered a variety of
theoretical developments. Those have to deal with completely different
situations, from atomically resolved phonon mapping to electron circular
dichroism passing by surface plasmon mapping. All of them rely on very
different physical approximations and have not yet been reconciled, despite
early attempts to do so. As an effort in that direction, we report on the
development of a scalar relativistic quantum electrodynamic (QED) approach of
the inelastic scattering of fast electrons. This theory can be adapted to
describe all modern EELS experiments, and under the relevant approximations,
can be reduced to any of the last EELS theories. In that aim, we present in
this paper the state of the art and the basics of scalar relativistic QED
relevant to the electron inelastic scattering. We then give a clear relation
between the two once antagonist descriptions of the EELS, the retarded green
Dyadic, usually applied to describe photonic excitations and the quasi-static
mixed dynamic form factor (MDFF), more adapted to describe core electronic
excitations of material. We then use this theory to establish two important
EELS-related equations. The first one relates the spatially resolved EELS to
the imaginary part of the photon propagator and the incoming and outgoing
electron beam wavefunction, synthesizing the most common theories developed for
analyzing spatially resolved EELS experiments. The second one shows that the
evolution of the electron beam density matrix is proportional to the mutual
coherence tensor, proving that quite universally, the electromagnetic
correlations in the target are imprinted in the coherence properties of the
probing electron beam.Comment: Re-Submission to SciPost. Updated version: minor revisions, SciPost
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Plasmonic Oligomers with Tunable Conductive Nanojunctions
International audienceEngineering plasmonic hot-spots is essential for applications of plasmonic nanoparticles. A particularly appealing route is to weld plasmonic nanoparticles together to form more complex structures sustaining plasmons with symmetries targeted to given applications. However, thecontrol of the welding and subsequent hotspot characteristic is still challenging. Herein, we demonstrate an original method that connects gold particles to their neighbors by another metal of choice. We first assemble gold bipyramids in a tip-to-tip configuration, yielding short chainsof variable length and grow metallic junctions in a second step. We follow the chain formation and the deposition of the second metal (i.e. silver or palladium) via UV/Vis spectroscopy and we map the plasmonic properties using electron energy loss spectroscopy. The formation ofsilver bridges leads to a huge redshift of the longitudinal plasmon modes into the mid-infrared region, while the addition of palladium results in a redshift accompanied by significant plasmon damping
Excitons and stacking order in h-BN
The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN)
makes this material one of the most promising candidate for light emitting
devices in the far ultraviolet (UV). However, single excitons occur only in
perfect monocrystals that are extremely hard to synthesize, while regular h-BN
samples present a complex emission spectrum with several additional peaks. The
microscopic origin of these additional emissions has not yet been understood.
In this work we address this problem using an experimental and theoretical
approach that combines nanometric resolved cathodoluminescence, high resolution
transmission electron microscopy and state of the art theoretical spectroscopy
methods. We demonstrate that emission spectra are strongly inhomogeneus within
individual flakes and that additional excitons occur at structural
deformations, such as faceted plane folds, that lead to local changes of the
h-BN stacking order
Visualizing plasmon-exciton polaritons at the nanoscale using electron microscopy
Polaritons are compositional light-matter quasiparticles that have recently
enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in
material science. Despite the enormous progress, however, a direct
nanometer-scale visualization of polaritons has remained an open challenge.
Here, we demonstrate that plasmon-exciton polaritons, or plexcitons, generated
by a hybrid system composed of an individual silver nanoparticle and a
few-layer transition metal dichalcogenide can be spectroscopically mapped with
nanometer spatial resolution using electron energy loss spectroscopy in a
scanning transmission electron microscope. Our experiments reveal important
insights about the coupling process, which have not been reported so far. These
include nanoscale variation of Rabi splitting and plasmon-exciton detuning, as
well as absorption-dominated extinction signals, which in turn provide the
ultimate evidence for the plasmon-exciton hybridization in the strong coupling
regime. These findings pioneer new possibilities for in-depth studies of
polariton-related phenomena with nanometer spatial resolution
Far-Field Radiation of Three-Dimensional Plasmonic Gold Tapers near Apexes
International audienceThree-dimensional plasmonic gold tapers are widely used structures in nano-optics for achieving imaging at the nanometer scale, enhanced spectroscopy, confined light sources, and ultrafast photoelectron emission. To understand their radiation properties further, especially in the proximity of the apex at the nanoscale, we employ cathodoluminescence spectroscopy with high spatial and energy resolution. The plasmon-induced radiation in the visible spectral range from three-dimensional gold tapers with opening angles of 13°and 47°is investigated under local electron excitation. We observe a much weaker radiation from the apex of the 13°taper than from that of the 47°taper. By means of finite-difference time-domain simulations we show that for small opening angles plasmon modes that are created at the apex are efficiently guided along the taper shaft. In contrast for tapers with larger opening angles, generated plasmon polaritons experience larger radiation damping. Interestingly, we find for both tapers that the most intense radiation comes from locations a few hundreds of nanometers behind the apexes, instead of exactly at the apexes. Our findings provide useful details for the design of plasmonic gold tapers as confined light sources or light absorbers
Design and implementation of a device based on an off-axis parabolic mirror to perform luminescence experiments in a scanning tunneling microscope
We present the design, implementation, and illustrative results of a light
collection/injection strategy based on an off-axis parabolic mirror collector
for a low-temperature Scanning Tunneling Microscope (STM). This device allows
us to perform STM induced Light Emission (STM-LE) and Cathodoluminescence
(STM-CL) experiments and in situ Photoluminescence (PL) and Raman spectroscopy
as complementary techniques. Considering the \'Etendue conservation and using
an off-axis parabolic mirror, it is possible to design a light collection and
injection system that displays 72% of collection efficiency (considering the
hemisphere above the sample surface) while maintaining high spectral resolution
and minimizing signal loss. The performance of the STM is tested by atomically
resolved images and scanning tunneling spectroscopy results on standard sample
surfaces. The capabilities of our system are demonstrated by performing STM-LE
on metallic surfaces and two-dimensional semiconducting samples, observing both
plasmonic and excitonic emissions. In addition, we carried out in situ PL
measurements on semiconducting monolayers and quantum dots and in situ Raman on
graphite and hexagonal boron nitride (h-BN) samples. Additionally, STM-CL and
PL were obtained on monolayer h-BN gathering luminescence spectra that are
typically associated with intragap states related to carbon defects. The
results show that the flexible and efficient light injection and collection
device based on an off-axis parabolic mirror is a powerful tool to study
several types of nanostructures with multiple spectroscopic techniques in
correlation with their morphology at the atomic scale and electronic structure.Comment: 19 pages, 14 Figure
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