396 research outputs found
Vibrational surface EELS probes confined Fuchs-Kliewer modes
Recently, two reports have demonstrated the amazing possibility to probe
vibrational excitations from nanoparticles with a spatial resolution much
smaller than the corresponding free-space phonon wavelength using electron
energy loss spectroscopy (EELS). While Lagos et al. evidenced a strong spatial
and spectral modulation of the EELS signal over a nanoparticle, Krivanek et al.
did not. Here, we show that discrepancies among different EELS experiments as
well as their relation to optical near- and far-field optical experiments can
be understood by introducing the concept of confined bright and dark
Fuchs-Kliewer modes, whose density of states is probed by EELS. Such a concise
formalism is the vibrational counterpart of the broadly used formalism for
localized surface plasmons; it makes it straightforward to predict or interpret
phenomena already known for localized surface plasmons such as
environment-related energy shifts or the possibility of 3D mapping of the
related surface charge densities
Probing the photonic local density of states with electron energy loss spectroscopy
Electron energy-loss spectroscopy (EELS) performed in transmission electron
microscopes is shown to directly render the photonic local density of states
(LDOS) with unprecedented spatial resolution, currently below the nanometer.
Two special cases are discussed in detail: (i) 2D photonic structures with the
electrons moving along the translational axis of symmetry and (ii) quasi-planar
plasmonic structures under normal incidence. Nanophotonics in general and
plasmonics in particular should benefit from these results connecting the
unmatched spatial resolution of EELS with its ability to probe basic optical
properties like the photonic LDOS.Comment: 4 pages, 2 figure
Probing Quantum Optical Excitations with Fast Electrons
Probing optical excitations with nanometer resolution is important for
understanding their dynamics and interactions down to the atomic scale.
Electron microscopes currently offer the unparalleled ability of rendering
spatially-resolved electron spectra with combined meV and sub-nm resolution,
while the use of ultrafast optical pulses enables fs temporal resolution and
exposure of the electrons to ultraintense confined optical fields. Here, we
theoretically investigate fundamental aspects of the interaction of fast
electrons with localized optical modes that are made possible by these
advances. We use a quantum-optics description of the optical field to predict
that the resulting electron spectra strongly depend on the statistics of the
sample excitations (bosonic or fermionic) and their population (Fock, coherent,
or thermal), whose autocorrelation functions are directly retrieved from the
ratios of electron gain intensities. We further explore feasible experimental
scenarios to probe the quantum characteristics of the sampled excitations and
their populations.Comment: 13 pages, 6 figures, 56 reference
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
Very low shot noise in carbon nanotubes
We have performed noise measurements on suspended ropes of single wall carbon
nanotubes (SWNT) between 1 and 300 K for different values of dc current through
the ropes. We find that the shot noise is suppressed by more than a factor 100
compared to the full shot noise 2eI. We have also measured an individual SWNT
and found a level of noise which is smaller than the minimum expected. Another
finding is the very low level of 1/f noise, which is significantly lower than
previous observations. We propose two possible interpretations for this strong
shot noise reduction: i) Transport within a rope takes place through a few
nearly ballistic tubes within a rope and possibly involves non integer
effective charges. ii) A substantial fraction of the tubes conduct with a
strong reduction of effective charge (by more than a factor 50).Comment: Submitted to Eur. Phys. J. B (Jan. 2002) Higher resolution pictures
are posted on http://www.lps.u-psud.fr/Collectif/gr_07/publications.htm
High-angular-resolution electron energy loss spectroscopy of hexagonal boron nitride
High-angular-resolution electron energy loss spectroscopy (EELS) is used to study the anisotropic behavior of the boron and nitrogen K ionization edges in h-BN. This work makes significant progress toward improving the anisotropy measurements. The authors show experimentally by EELS the vanishment of the p* peak existing in these K edges in agreement with electronic structure calculations and previous soft x-ray absorption spectroscopy measurements
Superconductivity in ropes of carbon nanotubes
Recent experimental and theoretical results on intrinsic superconductivity in
ropes of single-wall carbon nanotubes are reviewed and compared. We find strong
experimental evidence for superconductivity when the distance between the
normal electrodes is large enough. This indicates the presence of attractive
phonon-mediated interactions in carbon nanotubes, which can even overcome the
repulsive Coulomb interactions. The effective low-energy theory of rope
superconductivity explains the experimental results on the
temperature-dependent resistance below the transition temperature in terms of
quantum phase slips. Quantitative agreement with only one fit parameter can be
obtained. Nanotube ropes thus represent superconductors in an extreme 1D limit
never explored before.Comment: 19 pages, 9 figures, to appear in special issue of Sol. State Com
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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