64 research outputs found
Compact femtosecond electron diffractometer with 100 keV electron bunches approaching the single-electron pulse duration limit
We present the design and implementation of a highly compact femtosecond
electron diffractometer working at electron energies up to 100 keV. We use a
multi-body particle tracing code to simulate electron bunch propagation through
the setup and to calculate pulse durations at the sample position. Our
simulations show that electron bunches containing few thousands of electrons
per bunch are only weakly broadened by space-charge effects and their pulse
duration is thus close to the one of a single-electron wavepacket. With our
compact setup we can create electron bunches containing up to 5000 electrons
with a pulse duration below 100 femtoseconds on the sample. We use the
diffractometer to track the energy transfer from photoexcited electrons to the
lattice in a thin film of titanium. This process takes place on the timescale
of few-hundred femtoseconds and a fully equilibrated state is reached within
one picosecond.Comment: 5 pages, 3 figure
Femtosecond electrons probing currents and atomic structure in nanomaterials
The investigation of ultrafast electronic and structural dynamics in
low-dimensional systems like nanowires and two-dimensional materials requires
femtosecond probes providing high spatial resolution and strong interaction
with small volume samples. Low-energy electrons exhibit large scattering cross
sections and high sensitivity to electric fields, but their pronounced
dispersion during propagation in vacuum so far prevented their use as
femtosecond probe pulses in time-resolved experiments. Employing a
laser-triggered point-like source of either divergent or collimated electron
wave packets, we developed a hybrid approach for femtosecond point projection
microscopy and femtosecond low-energy electron diffraction. We investigate
ultrafast electric currents in nanowires with sub-100 femtosecond temporal and
few 10 nm spatial resolutions and demonstrate the potential of our approach for
studying structural dynamics in crystalline single-layer materials.Comment: 18 pages, 4 figures, includes 8 pages supplementary informatio
Symmetry-guided nonrigid registration: the case for distortion correction in multidimensional photoemission spectroscopy
Image symmetrization is an effective strategy to correct symmetry distortion
in experimental data for which symmetry is essential in the subsequent
analysis. In the process, a coordinate transform, the symmetrization transform,
is required to undo the distortion. The transform may be determined by image
registration (i.e. alignment) with symmetry constraints imposed in the
registration target and in the iterative parameter tuning, which we call
symmetry-guided registration. An example use case of image symmetrization is
found in electronic band structure mapping by multidimensional photoemission
spectroscopy, which employs a 3D time-of-flight detector to measure electrons
sorted into the momentum (, ) and energy () coordinates. In
reality, imperfect instrument design, sample geometry and experimental settings
cause distortion of the photoelectron trajectories and, therefore, the symmetry
in the measured band structure, which hinders the full understanding and use of
the volumetric datasets. We demonstrate that symmetry-guided registration can
correct the symmetry distortion in the momentum-resolved photoemission
patterns. Using proposed symmetry metrics, we show quantitatively that the
iterative approach to symmetrization outperforms its non-iterative counterpart
in the restored symmetry of the outcome while preserving the average shape of
the photoemission pattern. Our approach is generalizable to distortion
corrections in different types of symmetries and should also find applications
in other experimental methods that produce images with similar features
Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe
We investigate the interactions of photoexcited carriers with lattice
vibrations in thin films of the layered transition metal dichalcogenide (TMDC)
WSe. Employing femtosecond electron diffraction with monocrystalline
samples and first principle density functional theory calculations, we obtain a
momentum-resolved picture of the energy-transfer from excited electrons to
phonons. The measured momentum-dependent phonon population dynamics are
compared to first principle calculations of the phonon linewidth and can be
rationalized in terms of electronic phase-space arguments. The relaxation of
excited states in the conduction band is dominated by intervalley scattering
between valleys and the emission of zone-boundary phonons.
Transiently, the momentum-dependent electron-phonon coupling leads to a
non-thermal phonon distribution, which, on longer timescales, relaxes to a
thermal distribution via electron-phonon and phonon-phonon collisions. Our
results constitute a basis for monitoring and predicting out of equilibrium
electrical and thermal transport properties for nanoscale applications of
TMDCs
On the Role of Nuclear Motion in Singlet Exciton Fission: The Case of Single-Crystal Pentacene
Singlet exciton fission (SF), the formation of two triplet excitons from one singlet exciton, involves electronic, nuclear, and spin degrees of freedom as well as their couplings. Despite almost 60 years of research on this process, a complete microscopic understanding is still missing. One important open question concerns the role of nuclear motion in SF. In this perspective, recent results on the exciton dynamics are related to the structural dynamics of single-crystal pentacene and how they provide insights into that open question is shown. To probe the electronic dynamics, orbital-resolved measurements of the electronic structure are carried out using time- and angle-resolved photoemission spectroscopy. With femtosecond electron diffraction and with ab initio computations, the complementary nuclear dynamics is tracked. The results from both techniques are summarized, and how they relate to each other is discussed. Then, remaining open questions are outlined and potential routes are identified to tackle them, hopefully guiding future studies
Time-Domain Separation of Optical Properties From Structural Transitions in Resonantly Bonded Materials
The extreme electro-optical contrast between crystalline and amorphous states
in phase change materials is routinely exploited in optical data storage and
future applications include universal memories, flexible displays,
reconfigurable optical circuits, and logic devices. Optical contrast is
believed to arise due to a change in crystallinity. Here we show that the
connection between optical properties and structure can be broken. Using a
unique combination of single-shot femtosecond electron diffraction and optical
spectroscopy, we simultaneously follow the lattice dynamics and dielectric
function in the phase change material Ge2Sb2Te5 during an irreversible state
transformation. The dielectric function changes by 30% within 100 femtoseconds
due to a rapid depletion of electrons from resonantly-bonded states. This
occurs without perturbing the crystallinity of the lattice, which heats with a
2 ps time constant. The optical changes are an order-of-magnitude larger than
those achievable with silicon and present new routes to manipulate light on an
ultrafast timescale without structural changes
Photoinduced ultrafast transition of the local correlated structure in chalcogenide phase-change materials
Revealing the bonding and time-evolving atomic dynamics in functional
materials with complex lattice structures can update the fundamental knowledge
on rich physics therein, and also help to manipulate the material properties as
desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5
has been widely used in optical data storage and non-volatile electric memory
due to the fast switching speed and the low energy consumption. However, the
basic understanding of the structural dynamics on the atomic scale is still not
clear. Using femtosecond electron diffraction, structure factor calculation and
TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the
local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The
randomly oriented Peierls distortion among unit cells in the averaged rock-salt
phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast
suppression of the local Peierls distortions in individual unit cell gives rise
to a local structure change from the rhombohedral to the cubic geometry within
~ 0.3 ps. In addition, the impact of the carrier relaxation and the large
amount of vacancies to the ultrafast structural response is quantified and
discussed. Our work provides new microscopic insights into contributions of the
local correlated structure to the transient structural and optical responses in
phase change materials. Moreover, we stress the significance of femtosecond
electron diffraction in revealing the local correlated structure in the subunit
cell and the link between the local correlated structure and physical
properties in functional materials with complex microstructures
A machine learning route between band mapping and band structure
The electronic band structure (BS) of solid state materials imprints the
multidimensional and multi-valued functional relations between energy and
momenta of periodically confined electrons. Photoemission spectroscopy is a
powerful tool for its comprehensive characterization. A common task in
photoemission band mapping is to recover the underlying quasiparticle
dispersion, which we call band structure reconstruction. Traditional methods
often focus on specific regions of interests yet require extensive human
oversight. To cope with the growing size and scale of photoemission data, we
develop a generic machine-learning approach leveraging the information within
electronic structure calculations for this task. We demonstrate its capability
by reconstructing all fourteen valence bands of tungsten diselenide and
validate the accuracy on various synthetic data. The reconstruction uncovers
previously inaccessible momentum-space structural information on both global
and local scales in conjunction with theory, while realizing a path towards
integrating band mapping data into materials science databases
Bloch Wavefunction Reconstruction using Multidimensional Photoemission Spectroscopy
Angle-resolved spectroscopy is the most powerful technique to investigate the
electronic band structure of crystalline solids. To completely characterize the
electronic structure of topological materials, one needs to go beyond band
structure mapping and probe the texture of the Bloch wavefunction in
momentum-space, associated with Berry curvature and topological invariants.
Because phase information is lost in the process of measuring photoemission
intensities, retrieving the complex-valued Bloch wavefunction from
photoemission data has yet remained elusive. In this Article, we introduce a
novel measurement methodology and observable in extreme ultraviolet
angle-resolved photoemission spectroscopy, based on continuous modulation of
the ionizing radiation polarization axis. By tracking the energy- and
momentum-resolved amplitude and phase of the photoemission modulation upon
polarization variation, we reconstruct the Bloch wavefunction of prototypical
semiconducting transition metal dichalcogenide 2H-WSe with minimal theory
input. This novel experimental scheme, which is articulated around the
manipulation of the photoionization transition dipole matrix element, in
combination with a simple tight-binding theory, is general and can be extended
to provide insights into the Bloch wavefunction of many relevant crystalline
solids.Comment: 11 pages, 5 figure
- …