Ultrafast Electron Research Articles

Calculation of rf-induced temporal jitter in ultrafast electron diffraction

A significant contribution to the temporal resolution of an ultrafast electron diffraction (UED) instrument is arrival time jitter caused by amplitude and phase variation of radio-frequency (rf) cavities. In this paper, we present a semianalytical approach for calculating rf-induced temporal jitter from klystron and rf cavity parameters. Our approach allows fast estimation of temporal jitter for MeV-UED beamlines and can serve as a virtual timing tool when shot-to-shot measurements of rf amplitude and phase jitters are available. A simulation study for the SLAC MeV-UED instrument is presented, and the temporal resolution of several beamline configurations is compared. Published by the American Physical Society 2025

Attosecond electron dynamics in solid-state systems

Abstract Attosecond science has revolutionized the study of ultrafast electron dynamics. Originally based on high-order harmonic generation from intense laser fields, it provided groundbreaking insights into physical processes occurring on the few- to sub-femtosecond time scales. From its initial focus on atomic and molecular systems, the field rapidly expanded to solid-state materials, uncovering phenomena with possible significant implications for information technology. This review focuses on some of the key experimental techniques that enable attosecond resolution in solid-state systems. We categorize them into four main groups: core-hole clock spectroscopy, photoemission, XUV-based all-optical techniques, and sub-cycle strong-field approaches. Together, these methods contributed to significant breakthroughs, such as elucidating the timing of photoemission from solids, possibly enabling the manipulation of the electro-optical properties of a crystal with light fields, and advancing our understanding of fundamental light-matter interactions. Their application to novel materials and the development of innovative, cutting-edge light sources and techniques, will define the future of attoscience in solids, setting the basis for profound advancements in both scientific understanding and technological innovation.

Time-resolved momentum microscopy with fs-XUV photons at high repetition rates with flexible energy and time resolution

Time-resolved momentum microscopy is an emerging technique based on photoelectron spectroscopy for characterizing ultrafast electron dynamics and the out-of-equilibrium electronic structure of materials in the entire Brillouin zone with high efficiency. In this article, we introduce a setup for time-resolved momentum microscopy based on an energy-filtered momentum microscope coupled to a custom-made high-harmonic generation photon source driven by a multi-100 kHz commercial Yb-ultrafast laser that delivers fs pulses in the extreme ultraviolet range. The laser setup includes a nonlinear pulse compression stage employing spectral broadening in a Herriott-type bulk-based multi-pass cell. This element allows flexible tuning of the driving pulse duration, providing a versatile time-resolved momentum microscopy setup featuring two operational modes designed to enhance either the energy or time resolution. We show the capabilities of the system by tracing ultrafast electron dynamics in the conduction band valleys of a bulk crystal of the 2D semiconductor WS2. Using uncompressed driving laser pulses, we demonstrate an energy resolution better than (107 ± 2) meV, while compressed pulses lead to a time resolution better than (48.8 ± 17) fs.

Correlated spin-wave generation and domain-wall oscillation in a topologically textured magnetic film.

Spin waves, or magnons, are essential for next-generation energy-efficient spintronics and magnonics. Yet, visualizing spin-wave dynamics at nanoscale and microwave frequencies remains a formidable challenge due to the lack of spin-sensitive, time-resolved microscopy. Here we report a breakthrough in imaging dipole-exchange spin waves in a ferromagnetic film owing to the development of laser-free ultrafast Lorentz electron microscopy, which is equipped with a microwave-mediated electron pulser for high spatiotemporal resolution. Using topological spin textures, we captured the emission, propagation, reflection and interference of spin waves from spin anti-vortices under radio-frequency excitations. Remarkably, we show that spin-wave generation is closely tied to the oscillatory motion of specific magnetic domain walls, providing the missing link between wave emission and wall dynamics near magnetic singularities. This work opens new possibilities in magnonics, offering a nanoscopic view of spin dynamics via transmission electron microscopy and enabling controlled excitation via radio-frequency fields for exploring non-equilibrium states in magnetic and multiferroic systems.

Pulsed laser deposition assisted epitaxial growth of cesium telluride photocathodes for high brightness electron sources

The development of high-brightness electron sources is critical to state-of-the-art electron accelerator applications like X-ray free electron laser (XFEL) and ultra-fast electron microscopy. Cesium telluride is chosen as the electron source material for multiple cutting-edge XFEL facilities worldwide. This manuscript presents the first demonstration of the growth of highly crystalized and epitaxial cesium telluride thin films on 4H-SiC and graphene/4H-SiC substrates with ultrasmooth film surfaces. The ordering of the film was characterized by in situ reflection high energy electron diffraction and multiple X-ray diagnostics. The results of the quantum efficiency performance for epitaxial cesium telluride photocathodes are also reported.

Lattice Anisotropy-Driven Reduction of Phonon Velocities in Black Phosphorus.

Phonon dynamics and transport determine how heat is utilized and dissipated in materials. In 2D systems for optoelectronics and thermoelectrics, the impact of nanoscale material structure on phonon propagation is central to controlling thermal conduction. Here, we directly observe in-plane coherent acoustic phonon propagation in black phosphorus (BP) using ultrafast electron microscopy. We identify a significant reduction of the group velocities in directions intermediate to the armchair and zigzag lattice directions. Using a machine learning-based model with an >8000 atom supercell, we find that this slowing results from the mixing of in-plane transverse and longitudinal acoustic phonons and is independent of broken symmetries of edge reconstructions. This work demonstrates how coherent phonon transport is sensitive to propagation direction in the lattice plane.

Modulation of electron correlation dynamics in atomic non-sequential double ionization by counter-rotating two-color circularly polarized laser fields.

We investigate the ultrafast electron correlation effects during non-sequential double ionization (NSDI) of argon subjected to a combined femtosecond field composed of counter-rotating two-color circularly polarized (TCCP) pulse laser using a 3D classical ensemble model (CEM). Our simulation results reveal that manipulation of the carrier-envelope phase (CEP) of the external driving field modulates the dynamical behavior of the two electrons, resulting in a notable sensitivity of their momentum distribution to the relative phase of two components of the counter-rotating TCCP field. Through inversion analysis, we uncover the capability to direct electrons toward a single direction, thereby facilitating focused ion-electron collisions on the attosecond timescale. Furthermore, we observe that adjusting the CEP exerts a substantial influence on the NSDI ionization process, modifying the relative contributions of the recollision-induced ionization (RII) mechanism and recollision excitation with subsequent ionization (RESI) mechanism.

Spatiotemporal observation of surface plasmon polariton mediated ultrafast demagnetization

Surface plasmons offer a promising avenue in the pursuit of swift and localized manipulation of magnetism for advanced magnetic storage and information processing technology. However, observing and understanding spatiotemporal interactions between surface plasmons and spins remains challenging, hindering optimal optical control of magnetism. Here, we demonstrate the spatiotemporal observation of patterned ultrafast demagnetization dynamics in permalloy mediated by propagating surface plasmon polaritons with sub-picosecond time- and sub-μm spatial- scales by employing Lorentz ultrafast electron microscopy combined with excitation through transient optical gratings. We discover correlated spatial distributions of demagnetization amplitude and surface plasmon polariton intensity, the latter characterized by photo-induced near-field electron microscopy. Furthermore, by comparing the results with patterned ultrafast demagnetization dynamics without surface plasmon polariton interaction, we show that the demagnetization is not only enhanced but also exhibits a spatiotemporal modulation near a spatial discontinuity (plasmonic hot spot). Our findings shed light on the intricate interplay between surface plasmons and spins, offer insights into the optimized control of optical excitation of magnetic materials and push the boundaries of ultrafast manipulation of magnetism.

Defects Calculation and Accelerated Interfacial Charge Transfer in a Photoactive MOF-Based Heterojunction.

Photocatalytic hydrogen production is currently considered a clean and sustainable route to meet the energy and environmental issues. Among, heterojunction photocatalysts have been developed to improve their photocatalytic efficiency. Defect engineering of heterojunction photocatalysts is attractive due to it can perform as electron trap and change the band structure to optimize the interfacial separation rate of photogenerated electron-hole pairs. Here, the MOF-based heterojunction photocatalysts with theoretically high reduction and oxidation abilities are successfully synthesized, denoted ZrO2/Pt/Zr-MOF-X, with tuned linker defectivity through an in situ electrochemical route. The defectivity are rationally calculated from the TG and 1H NMR results. A positive correlation is found between the defectivity and photocatalytic activity, and ZrO2/Pt/Zr-MOF-6 with the optimized defectivity of ca. 35% exhibits the highest hydrogen production rate of up to 2923 µmol g-1 h-1, illustrating the importance of structural defects in heterojunction photocatalysts. Ultrafast transient absorption spectroscopy and electron spin resonance results unveil the highest carrier concentration and charge separation efficiency in the defected heterostructure of ZrO2/Pt/Zr-MOF-6 through a direct Z-scheme contact, leading to its efficient photocatalysis through the high redox power.

Detection of ultrafast electron energization by whistler-mode chorus waves in the magnetosphere of Earth

Electromagnetic whistler-mode chorus waves are a key driver of variations in energetic electron fluxes in the Earth’s magnetosphere through the wave-particle interaction. Traditionally understood as a diffusive process, these interactions account for long-term electron flux variations (> several minutes). However, theories suggest that chorus waves can also cause rapid (< 1 s) electron acceleration and significant flux variations within less than a second through a nonlinear wave-particle interaction. Detecting these rapid accelerations has been a great challenge due to a limited time resolution of conventional particle instruments. Here, we employ an analysis technique to enhance the time resolution of the particle measurements, revealing rapid electron flux variations within less than one second associated with chorus waves. This technique exposes short-lived flux increases significantly larger than those observable with the standard time resolution. Our findings indicate that these transient flux variations result from the nonlinear acceleration of electrons induced by the chorus waves, highlighting the importance of nonlinear wave-particle interactions in creating high energy electrons in the Earth’s magnetosphere. The same acceleration mechanism should operate in the magnetospheres of Jupiter and Saturn where chorus waves are present, and in laboratory plasma environments when chorus-like waves are excited.

Influence of Photoemission Geometry on Timing and Efficiency in 4D Ultrafast Electron Microscopy.

Broader adoption of 4D ultrafast electron microscopy (UEM) for the study of chemical, materials, and quantum systems is being driven by development of new instruments as well as continuous improvement and characterization of existing technologies. Perhaps owing to the still-high barrier to entry, the full range of capabilities of laser-driven 4D UEM instruments has yet to be established, particularly when operated at extremely low beam currents (~fA). Accordingly, with an eye on beam stability, we have conducted particle tracing simulations of unconventional off-axis photoemission geometries in a UEM equipped with a thermionic-emission gun. Specifically, we have explored the impact of experimentally adjustable parameters on the time-of-flight (TOF), the collection efficiency (CE), and the temporal width of ultrashort photoelectron packets. The adjustable parameters include the Wehnelt aperture diameter (DW), the cathode set-back position (Ztip), and the position of the femtosecond laser on the Wehnelt aperture surface relative to the optic axis (Rphoto). Notable findings include significant sensitivity of TOF to DW and Ztip, as well as non-intuitive responses of CE and temporal width to varying Rphoto. As a means to improve accessibility, practical implications and recommendations are emphasized wherever possible.

Design for light-based spherical aberration correction of ultrafast electron microscopes.

We theoretically demonstrate that ponderomotive interactions near the electron cross-over can be used for aberration correction in ultrafast electron microscopes. Highly magnified electron shadow images from Si3N4 thin films are utilized to visualize the distortions induced by spherical aberrations. Our simulations of electron-light interactions indicate that spherical aberrations can be compensated resulting in an aberration-free angle of 8.1 mrad. For achieving the necessary light distribution, we use a gradient descent algorithm to optimize Zernike polynomials and shape the light beam into a modified Gaussian and Laguerre-Gaussian beam.

Temperature field simulation and experimental investigation for column-faced 45 steel via ultrafast electron beam scanning

Temperature field simulation and experimental investigation for column-faced 45 steel via ultrafast electron beam scanning

State-of-the-art electron beams for compact tools of ultrafast science

We review state-of-the-art electron beams for single-shot megaelectronvolt ultrafast electron diffraction (MeV-UED) and compact light sources. Our primary focus is on sub-100 femtosecond electron bunches in the 2-30 MeV energy range. We demonstrate that our new and recent simulation results permit significantly improved bunch parameters for these applications.

Comparative study of electron transport through aromatic molecules on gold nanoparticles: insights from soft X-ray spectroscopy of condensed nanoparticle films versus flat monolayer films.

Understanding electron transport in self-assembled monolayers on metal nanoparticles (NPs) is crucial for developing NP-based nanodevices. This study investigates ultrafast electron transport through aromatic molecules on NP surfaces via resonant Auger electron spectroscopy (RAES) with a core-hole-clock (CHC) approach. Aromatic molecule-coated Au NPs are deposited to form condensed NP films, and flat monolayers are prepared for comparison. Soft X-ray techniques, including X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, confirm oriented monolayers in both NP and flat films. The nuclear dynamics is studied via ion yield measurements. After subtracting secondary processes, the ion yield spectra of the condensed NP films reveal site-selective desorption of the methyl ester group by resonant core excitation. The ultrafast electron transport time from the carbonyl group through the phenyl rings to the metal surfaces in the condensed NP films is successfully determined via the RAES-CHC approach by subtracting inelastic scattering components. The chain length of the aromatic molecules influence the electron transport time in the NP films, reflecting the trends observed in the flat films. This evidence supports ultrafast electron transport via the through-bond model, independent of interactions between the molecules adsorbed on an NP itself or adjacent NPs. Identifying and subtracting background spectral components of the condensed NP films allows accurate analysis of the ultrafast dynamics. This study suggests that insights gained from electron transport processes in the flat monolayer films can be extrapolated to practical NP-molecule interfaces, providing valuable insights for the molecular design of NP-based devices.

Ultrafast structural dynamics of UV photoexcited cis,cis-1,3-cyclooctadiene observed with time-resolved electron diffraction.

Conjugated diene molecules are highly reactive upon photoexcitation and can relax through multiple reaction channels that depend on the position of the double bonds and the degree of molecular rigidity. Understanding the photoinduced dynamics of these molecules is crucial for establishing general rules governing the relaxation and product formation. Here, we investigate the femtosecond time-resolved photoinduced excited-state structural dynamics of cis,cis-1,3-cyclooctadiene, a large-flexible cyclic conjugated diene molecule, upon excitation with 200 nm using mega-electron-volt ultrafast electron diffraction and trajectory surface hopping dynamics simulations. We tracked the photoinduced structural changes from the Franck-Condon region through the conical intersection seam to the ground state. Our findings revealed a novel primary reaction coordinate involving ring distortion, where the ring stretches along one axis and compresses along the perpendicular axis. The nuclear wavepacket remains compact along this reaction coordinate until it reaches the conical intersection seam, and it rapidly spreads as it approaches the ground state, where multiple products are formed.

Nature of the carrier dynamics and contrast formation on the photoactive material surfaces: Insight from ultrafast imaging to DFT calculations.

Precise material design and surface engineering play a crucial role in enhancing the performance of optoelectronic devices. These efforts are undertaken to particularly control the optoelectronic properties and regulate charge carrier dynamics at the surface and interface. In this study, we used ultrafast scanning electron microscopy (USEM), which is a powerful and highly sensitive surface tool that provides unique information about the photoactive charge dynamics of material surfaces selectively and spontaneously in real time and space in high spatial and temporal resolution. Here, time-resolved images of CdTe (110), CdSe (100), GaAs (110), and other semiconductors revealed that the presence of oxide layers on the surface of materials leads to an increase in the work function (WF) and trap state densities upon optical excitation, leading to the formation of dark image contrast in USEM experiments. These findings were further supported by abinitio calculations, which confirmed the reliability of the observed changes in the excited surface WFs. Besides enhancing our understanding of surface charge dynamics, it also offers valuable insights into manipulating these properties. This study paves the way for precise control and the potential to design highly efficient light-harvesting devices.

Ultrafast Electron Dynamics in Coupled and Uncoupled HgTe Quantum Dots.

In this article, we study electron dynamics in HgTe quantum dots with a 1.9 μm gap, a material relevant for infrared sensing and emission, using ultrafast spectroscopy with 35 fs time resolution. Experiments have been carried out at several probing photon energies around the gap, which allows us to follow the relaxation path of the photoexcited electrons. We compare such dynamics in two kind of samples, HgTe quantum dots with long ligands and with short ligands, in order to distinguish the role of the coupling between adjacent quantum dots. Three main dynamics can be observed in the transient reflectivity on both samples, with slightly different relaxation times: two fast decays on the time scale of hundreds of femtoseconds and a few picoseconds, respectively, followed by a slower relaxation back to the unperturbed value over hundreds of picoseconds. The two fast components are associated with intraband relaxation of the photoexcited electrons within the conduction band, while the final relaxation path can be assigned to Auger relaxation mechanisms and to the slower interband exciton recombination.

Excimer formation in zinc-phthalocyanine revealed using ultrafast electron diffraction

The formation of excited dimer states, so called excimers, is an important phenomenon in many organic molecular semiconductor solid state aggregates. In contrast to Frenkel exciton-polarons, an excimer is long-lived and energetically low-lying due to stabilization resulting from a substantial reorganization of the intermolecular geometry. Here, we show that ultrafast electron diffraction can follow the dynamics of solid-state excimer formation in polycrystalline thin films of a molecular semiconductor, revealing both the key reaction modes and the eventual structure of the emitting state. We study the prototypical organic semiconductor zinc-phthalocyanine (ZnPc) in its crystallographic α-phase as a model excimeric system. We show that the excimer forms in a two-step process starting with a fast dimerization (approx. 0.4 ps) followed by a subsequent slow shear-twist motion (14 ps) leading to an alignment of the π-systems of the involved monomers. This structural distortion persists well beyond 300 ps. Furthermore, we show that while the same excimer geometry is present in partially fluorinated derivatives of ZnPc, the formation kinematics slow down with increasing level of fluorination.

Photocontrol of ferroelectricity in multiferroic BiFeO3 via structural modification coupled with photocarrier

Ultrafast control of ferroelectricity and magnetism by light is essential for future development in multiple functioning devices. Here, we demonstrate that the intense and ultrafast photo-modulation of the electric dipole can be realized by photocarrier injection into a multiferroic BiFeO3 thin film using optical pump-probe and second harmonic generation measurements. Results of ultrafast electron diffraction with <100 fs time resolution and theoretical study reveal that the localized photocarrier strongly couples with the lattice structure and becomes the origin for the observed sudden change in the electric dipole. In addition, the subsequent structural dynamics involve a strong oscillation with a frequency of ~3.3 THz despite a poor structural symmetry change. Based on a theoretical calculation, this oscillation can be attributed to an unexpectedly softened new phonon mode generated by mixing essential two phonon modes governing the multiferroic (ferroelectric and antiferromagnetic) nature of BiFeO3 in the ground state due to strong coupling with a localized photocarrier. The comprehensive study shows that injection of the localized photocarrier strongly coupled with the lattice vibration mode can simultaneously realize the ultrafast switching of electric dipoles and magnetic interaction at once, even at room temperature, without modifying the long-range lattice structure.