214 research outputs found
Solving the Jitter Problem in Microwave Compressed Ultrafast Electron Diffraction Instruments: Robust Sub-50 fs Cavity-Laser Phase Stabilization
We demonstrate the compression of electron pulses in a high-brightness
ultrafast electron diffraction (UED) instrument using phase-locked microwave
signals directly generated from a mode-locked femtosecond oscillator.
Additionally, a continuous-wave phase stabilization system that accurately
corrects for phase fluctuations arising in the compression cavity from both
power amplification and thermal drift induced detuning was designed and
implemented. An improvement in the microwave timing stability from 100 fs to 5
fs RMS is measured electronically and the long-term arrival time stability
(10 hours) of the electron pulses improves to below our measurement
resolution of 50 fs. These results demonstrate sub-relativistic ultrafast
electron diffraction with compressed pulses that is no longer limited by
laser-microwave synchronization.Comment: Accepted for publication in Structural Dynamic
Ultrafast Phonon-Diffuse Scattering as a Tool for Observing Chiral Phonons in Monolayer Hexagonal Lattices
At the 2D limit, hexagonal systems such as monolayer transition metal
dichalcogenides (TMDs) and graphene exhibit unique coupled spin and
momentum-valley physics (valley pseudospin) owing to broken spatial inversion
symmetry and strong spin-orbit coupling. Circularly polarized light provides
the means for pseudospin-selective excitation of excitons (or electrons and
holes) and can yield momentum-valley polarized populations of carriers that are
the subject of proposed valleytronic applications. The chirality of these
excited carriers have important consequences for the available
relaxation/scattering pathways, which must conserve (pseudo)angular momentum as
well as energy. One available relaxation channel that satisfies these
constraints is coupling to chiral phonons. Here we show that chiral
carrier-phonon coupling following valley-polarized photoexcitation is expected
to leads to a strongly valley-polarized chiral phonon distribution that is
directly measurable using ultrafast phonon-diffuse scattering techniques. Using
ab-initio calculations we show how the dynamic phonon occupations and valley
anisotropy determined by nonequilibrium observations can provide a new window
on the physical processes that drive carrier valley-depolarization in monolayer
TMDs
Ultrafast Electron Diffuse Scattering as a Tool for Studying Phonon Transport: Phonon Hydrodynamics and Second Sound Oscillations
Hydrodynamic phonon transport phenomena, like second sound, have been
observed in liquid Helium temperatures more than 50 years ago. More recently
second sound has been observed in graphite at over 200\,K using transient
thermal grating techniques. In this work we explore the signatures of second
sound in ultrafast electron diffuse scattering (UEDS) patterns. We use density
functional theory and solve the Boltzmann transport equation to determine
time-resolved non-equilibrium phonon populations and subsequently calculate
one-phonon structure factors and diffuse scattering patterns to simulate
experimental data covering the regimes of ballistic, diffusive, and
hydrodynamic phonon transport. For systems like graphite, UEDS is capable of
extracting time-dependent phonon occupancies across the entire Brillouin zone
and ultimately lead to a more fundamental understanding of the hydrodynamic
phonon transport regime.Comment: 7 pages, 4 figure
New electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range
We present a method for producing sub-100 fs electron bunches that are
suitable for single-shot ultrafast electron diffraction experiments in the 100
keV energy range. A combination of analytical results and state-of-the-art
numerical simulations show that it is possible to create 100 keV, 0.1 pC, 20 fs
electron bunches with a spotsize smaller than 500 micron and a transverse
coherence length of 3 nm, using established technologies in a table-top set-up.
The system operates in the space-charge dominated regime to produce
energy-correlated bunches that are recompressed by established radio-frequency
techniques. With this approach we overcome the Coulomb expansion of the bunch,
providing an entirely new ultrafast electron diffraction source concept
Generation of sub-fs electron beams at few-MeV energies
Time resolved electron diffraction is an alternative approach to FEL based X-ray experiments for the study of structural dynamics of matter on the relevant timescales. The required electron beam parameters are demanding in terms of emittance and bunch length and require the operation at charges typically well below 1 pC. Moreover the energy is low – a few MeV only. The longitudinal compression of the bunches can be realized with a simple longitudinal focusing scheme in a drift. In this paper the question of what limits the bunch length in this parameter regime is addressed by means of numerical simulations. Beside emittance increasing space charge effects also rf-curvature and nonlinear compression are identified as limiting factors
Ultrafast Electron Diffraction: Visualizing Dynamic States of Matter
Since the discovery of electron-wave duality, electron scattering instrumentation has developed into a powerful array of techniques for revealing the atomic structure of matter. Beyond detecting local lattice variations in equilibrium structures with the highest possible spatial resolution, recent research efforts have been directed towards the long sought-after dream of visualizing the dynamic evolution of matter in real-time. The atomic behavior at ultrafast timescales carries critical information on phase transition and chemical reaction dynamics, the coupling of electronic and nuclear degrees of freedom in materials and molecules, the correlation between structure, function and previously hidden metastable or nonequilibrium states of matter. Ultrafast electron pulses play an essential role in this scientific endeavor, and their generation has been facilitated by rapid technical advances in both ultrafast laser and particle accelerator technologies. This review presents a summary of the remarkable developments in this field over the last few decades. The physics and technology of ultrafast electron beams is presented with an emphasis on the figures of merit most relevant for ultrafast electron diffraction (UED) experiments. We discuss recent developments in the generation, manipulation and characterization of ultrashort electron beams aimed at improving the combined spatio-temporal resolution of these measurements. The fundamentals of electron scattering from atomic matter and the theoretical frameworks for retrieving dynamic structural information from solid-state and gas-phase samples is described. Essential experimental techniques and several landmark works that have applied these approaches are also highlighted to demonstrate the widening applicability of these methods. Ultrafast electron probes with ever improving capabilities, combined with other complementary photon-based or spectroscopic approaches, hold tremendous potential for revolutionizing our ability to observe and understand energy and matter at atomic scales
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