Dynamic topological domain walls driven by lithium intercalation in graphene.

Stacking engineering in van der Waals (vdW) materials is a powerful method to control topological electronic phases for quantum device applications. Atomic intercalation into the vdW material can modulate the stacking structure at the atomic scale without a highly technical protocol. Here we report that lithium intercalation in a topologically structured graphene/buffer system on SiC(0001) drives dynamic topological domain wall (TDW) motions associated with stacking order change by using an in situ aberration-corrected low-energy electron microscope in combination with theoretical modelling. We observe sequential and selective lithium intercalation that starts at topological crossing points (AA stacking) and then selectively extends to AB stacking domains. Lithium intercalation locally changes the domain stacking order to AA and in turn alters the neighbouring TDW stacking orders, and continuous intercalation drives the evolution of the whole topological structure network. Our work reveals moving TDWs protected by the topology of stacking and lays the foundation for controlling the stacking structure via atomic intercalation. These findings open up new avenues to realize intercalation-driven vdW electronic devices.

Toward fully automated UED operation using two-stage machine learning model.

To demonstrate the feasibility of automating UED operation and diagnosing the machine performance in real time, a two-stage machine learning (ML) model based on self-consistent start-to-end simulations has been implemented. This model will not only provide the machine parameters with adequate precision, toward the full automation of the UED instrument, but also make real-time electron beam information available as single-shot nondestructive diagnostics. Furthermore, based on a deep understanding of the root connection between the electron beam properties and the features of Bragg-diffraction patterns, we have applied the hidden symmetry as model constraints, successfully improving the accuracy of energy spread prediction by a factor of five and making the beam divergence prediction two times faster. The capability enabled by the global optimization via ML provides us with better opportunities for discoveries using near-parallel, bright, and ultrafast electron beams for single-shot imaging. It also enables directly visualizing the dynamics of defects and nanostructured materials, which is impossible using present electron-beam technologies.

A synchrotron-based kilowatt-level radiation source for EUV lithography.

A compact damping ring with a limited circumference of about 160 m is proposed for producing kilowatt-level coherent EUV radiation. The electron bunch in the storage ring is modulated by a 257 nm wavelength seed laser with the help of the angular-dispersion-induced micro-bunching method (Feng and Zhao in Sci Rep 7:4724, 2017), coherent radiation at 13.5 nm with an average power of about 2.5 kW can be achieved with the state-of-the-art accelerator and laser technologies.

Accurate prediction of mega-electron-volt electron beam properties from UED using machine learning.

To harness the full potential of the ultrafast electron diffraction (UED) and microscopy (UEM), we must know accurately the electron beam properties, such as emittance, energy spread, spatial-pointing jitter, and shot-to-shot energy fluctuation. Owing to the inherent fluctuations in UED/UEM instruments, obtaining such detailed knowledge requires real-time characterization of the beam properties for each electron bunch. While diagnostics of these properties exist, they are often invasive, and many of them cannot operate at a high repetition rate. Here, we present a technique to overcome such limitations. Employing a machine learning (ML) strategy, we can accurately predict electron beam properties for every shot using only parameters that are easily recorded at high repetition rate by the detector while the experiments are ongoing, by training a model on a small set of fully diagnosed bunches. Applying ML as real-time noninvasive diagnostics could enable some new capabilities, e.g., online optimization of the long-term stability and fine single-shot quality of the electron beam, filtering the events and making online corrections of the data for time-resolved UED, otherwise impossible. This opens the possibility of fully realizing the potential of high repetition rate UED and UEM for life science and condensed matter physics applications.

High order phase contrast and source divergence in low energy electron microscopy.

We present experimental observations of high order phase contrast in aberration corrected low energy electron microscopy (AC-LEEM). Phase contrast produced by atomic steps on a Ag (111) surface exhibits prominent high order interference fringes, which have not been reported before. These phase contrast features depend upon defocus and incident electron energy, similar to the prominent first order fringes observed previously and in agreement with Fourier optics (FO) model predictions. The comparison of experimental results and FO model simulations demonstrates that fringe amplitudes are strongly affected at large defocus by the source divergence. This effect is exploited to quantitatively determine the divergence, 0.055 ± 0.005 mrad, of the field emission source in AC-LEEM under the imaging conditions used. Although the divergence determines the spatial coherence of the illumination in microscopy, it has not been possible to characterize this key instrumental parameter in LEEM before.

High-order exceptional point based optical sensor.

Exceptional points (EPs) could potentially enhance the sensitivity of an optical sensing system by orders of magnitude. Higher-order EP systems, having more complex physics, can further boost this parameter. In this paper, we investigate the response order of high-order non-Hermitian systems and provide a guideline for designing a sensor with high response order. Based on this design rule, we propose and demonstrate an optical sensor with a fourth-order response, and analyze its associated properties. The four resonant wavelengths of our optical sensor simultaneously collapse at a high-order exceptional point in the parameter space, providing a fourth root relation between the amount of wavelength splitting and the amplitude of the perturbation. A large sensitivity enhancement factor over 100, is observed when the wavelength splitting is compared with traditional single resonator-based sensors under small perturbation conditions.

Systematic analysis of a compact setup to measure the photoemitted electron beam transverse momentum and emittance.

A compact setup with a planar-cathode and grid-anode plus free field drift distance configuration (momentatron) has provided a new way to measure the transverse momentum and, hence, the emittance of the electron beam from a photocathode. This method has been used for analysis of the transverse momentum and emittance of the photoemitted electron beam from the photocathode in a stepwise manner during the fabrication process. The errors caused by the lensing effect from opening holes of the grid anode and misalignments caused by tilting and curving have been systematically analyzed. An analytical method has been developed, and a full three-dimensional electrostatic field particle tracing simulation has been performed to validate this measurement technique. The results show that a momentatron can provide an accurate measurement of transverse momentum and emittance of the photoemitted electrons. The reasonable experimental errors that may be encountered will only have a modest (few %) effect on the emittance measurement.

Toward monochromated sub-nanometer UEM and femtosecond UED.

A preliminary design of a mega-electron-volt (MeV) monochromator with 10-5 energy spread for ultrafast electron diffraction (UED) and ultrafast electron microscopy (UEM) is presented. Such a narrow energy spread is advantageous in both the single shot mode, where the momentum resolution in diffraction is improved, and the accumulation mode, where shot-to-shot energy jitter is reduced. In the single-shot mode, we numerically optimized the monochromator efficiency up to 13% achieving 1.3 million electrons per pulse. In the accumulation mode, to mitigate the efficiency degradation caused by the shot-to-shot energy jitter, an optimized gun phase yields only a mild reduction of the single-shot efficiency, therefore the number of accumulated electrons nearly proportional to the repetition rate. Inspired by the recent work of Qi et al. (Phys Rev Lett 124:134803, 2020), a novel concept of applying reverse bending magnets to adjust the energy-dependent path length difference has been successfully realized in designing a MeV monochromator to achieve the minimum energy-dependent path length difference between cathode and sample. Thanks to the achromat design, the pulse length of the electron bunches and the energy-dependent timing jitter can be greatly reduced to the 10 fs level. The introduction of such a monochromator provides a major step forward, towards constructing a UEM with sub-nm resolution and a UED with ten-femtosecond temporal resolution. The one-to-one mapping between the electron beam parameter and the diffraction peak broadening enables a real-time nondestructive diagnosis of the beam energy spread and divergence. The tunable electric-magnetic monochromator allows the scanning of the electron beam energy with a 10-5 precision, enabling online energy matching for the UEM, on-momentum flux maximizing for the UED and real-time energy measuring for energy-loss spectroscopy. A combination of the monochromator and a downstream chicane enables "two-color" double pulses with femtosecond duration and the tunable delay in the range of 10 to 160 fs, which can potentially provide an unprecedented femtosecond time resolution for time resolved UED.

Aberration corrected spin polarized low energy electron microscope.

Spin Polarized Low Energy Electron Microscopy (SPLEEM) is a powerful tool to reveal the magnetic structure of ferromagnetic surfaces on the atomic depth scale level[1-3]. With aberration corrected LEEM and a high brightness spin polarized electron gun, high spatial resolution will provide more details for ultra-thin ferromagnetic film studies. This study reports the first realization of aberration corrected SPLEEM (AC-SPLEEM). The performance of the setup was tested on ferromagnetic Fe nanoscale islands on a W(110) single crystal, with spatial resolution of 3.3 nm in spin asymmetry images.

Breaking 50 Femtosecond Resolution Barrier in MeV Ultrafast Electron Diffraction with a Double Bend Achromat Compressor.

We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20 fC beam after bunch compression are measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6 THz corresponding to Bismuth A_{1g} mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications.

A novel nondestructive diagnostic method for mega-electron-volt ultrafast electron diffraction.

A real-time, nondestructive, Bragg-diffracted electron beam energy, energy-spread and spatial-pointing jitter monitor is experimentally verified by encoding the electron beam energy and spatial-pointing jitter information into the mega-electron-volt ultrafast electron diffraction pattern. The shot-to-shot fluctuation of the diffraction pattern is then decomposed to two basic modes, i.e., the distance between the Bragg peaks as well as its variation (radial mode) and the overall lateral shift of the whole pattern (drift mode). Since these two modes are completely decoupled, the Bragg-diffraction method can simultaneously measure the shot-to-shot energy fluctuation from the radial mode with 2·10-4 precision and spatial-pointing jitter from the drift mode having wide measurement span covering energy jitter range from 10-4 to 10-1. The key advantage of this method is that it allows us to extract the electron beam energy spread concurrently with the ongoing experiment and enables online optimization of the electron beam especially for future high charge single-shot ultrafast electron diffraction (UED) and ultrafast electron microscopy (UEM) experiments. Furthermore, real-time energy measurement enables the filtering process to remove off-energy shots, improving the resolution of time-resolved UED. As a result, this method can be applied to the entire UED user community, beyond the traditional electron beam diagnostics of accelerators used by accelerator physicists.

A compact tunable quadrupole lens for brighter and sharper ultra-fast electron diffraction imaging.

In this article, we report our proof-of-principle design and experimental commissioning of a broadly tunable and low-cost transverse focusing lens system for MeV-energy electron beams. The lens system based on electromagnetic (EM) quadrupoles has been built as a part of the existing instrument for ultra-fast electron diffraction (UED) experiments at the Accelerator Test Facility II (ATF-II) at Brookhaven National Laboratory (BNL). We experimentally demonstrated the independent control of the size and divergence of the beam with the charge ranging from 1 to 13 pC. The charge density and divergence of the beam at the sample are the most important factors determining the quality of the Bragg-diffraction image (BDI). By applying the Robust Conjugate Directional Search (RCDS) algorithm for online optimization of the quadrupoles, the transverse beam size can be kept constant down to 75 µm from 1 to 13 pC. The charge density is nearly two orders of magnitude higher than the previously achieved value using a conventional solenoid. Using the BDI method we were able to extract the divergence of the beam in real-time and apply it to the emittance measurement for the first time. Our results agree well with simulations and with the traditional quadrupole scan method. The real-time divergence measurement opens the possibility of online optimization of the beam divergence (<0.2 mrad) at the sample with the increased beam charge. This optimization is crucial for the future development of single-shot ultra-fast electron microscope (UEM). Finally, we demonstrated BDI with significant improvement, up to 3 times higher peak intensity and 2 times sharper Bragg-diffraction peaks at 13 pC. The charge is now limited by the laser power and increasing charge may improve the quality of BDI further. The capability we demonstrated here provides us with opportunities for new sciences using near-parallel, bright and ultrafast electron beams for single-shot imaging, to directly visualize the dynamics of defects and nanostructured materials, or even record molecular movie, which are impossible using present electron-beam technologies.

Coverage-driven phase transition of copper silicide monolayer on Si (111).

The characterization and control of atomic substitution process is crucial in fabricating high-quality two-dimensional layered compound materials and tuning their physical properties. With intensity-voltage low energy electron microscopy (IV-LEEM), we found that the concentration of copper in the topmost copper silicide monolayer on Si (111) substrates varies gradually from 1.7 to 1.0 ML while preserving it's unique '5 × 5' incommensurate phase in a transition region as large as 1000 nm. This gradual variation of the copper concentration is due to the incomplete substitution of the Si with Cu, as revealed by atomic-resolved scanning tunneling microscopy with a tip that nicely resolved the '5 × 5' periodicity. Our experiments indicate that besides the widely-accepted phase of Cu2Si with both substitutional and interstitial Cu atoms, another type of precursor copper silicide CuSi3 with only interstitial Cu atoms also plays important roles in the substitutional diffusion and reaction processes during the formation of the topmost copper silicide monolayer. This precursor phase might exist in the growth of other two-dimensional layered materials with potential applications in integrated optoelectronics, spintronics or low dissipative devices.

Design of compact ultrafast microscopes for single- and multi-shot imaging with MeV electrons.

Ultrafast high-energy electron microscopy, taking advantage of strong interaction of electrons with matter while minimizing space charge problems, can be used to address a wide range of grand challenges in basics energy sciences. However, MeV-electron lenses are inherently bulky and expensive, preventing them from acceptance in a broad scientific community. In this article, we report our novel design of a compact, low-cost imaging-lens system for MeV-electrons based on quadrupole multiplets, including triplet, quadruplet and quintuplet, both symmetric and asymmetric. We compare optical performance of quadrupole-based condenser, objective and projector lenses with that of the traditional round-lenses and discuss the strategy for their practical use in constructing MeV-electron microscopes for high spatial and temporal resolution single- and multi-shot imaging. Combining the compound electron-optical system with a photocathode radiofrequency (RF) gun, such a MeV electron microscope can be fit into a small-sized laboratory for ultrafast observations and measurements.

Space charge effects and aberrations on electron pulse compression in a spherical electrostatic capacitor.

The effects of space charge, aberrations and relativity on temporal compression are investigated for a compact spherical electrostatic capacitor (α-SDA). By employing the three-dimensional (3D) field simulation and the 3D space charge model based on numerical General Particle Tracer and SIMION, we map the compression efficiency for a wide range of initial beam size and single-pulse electron number and determine the optimum conditions of electron pulses for the most effective compression. The results demonstrate that both space charge effects and aberrations prevent the compression of electron pulses into the sub-ps region if the electron number and the beam size are not properly optimized. Our results suggest that α-SDA is an effective compression approach for electron pulses under the optimum conditions. It may serve as a potential key component in designing future time-resolved electron sources for electron diffraction and spectroscopy experiments.

Reduction of Intrinsic Electron Emittance from Photocathodes Using Ordered Crystalline Surfaces.

The generation of intense electron beams with low emittance is key to both the production of coherent x rays from free electron lasers, and electron pulses with large transverse coherence length used in ultrafast electron diffraction. These beams are generated today by photoemission from disordered polycrystalline surfaces. We show that the use of single crystal surfaces with appropriate electronic structures allows us to effectively utilize the physics of photoemission to generate highly directed electron emission, thus reducing the emittance of the electron beam being generated.

Design and commissioning of an aberration-corrected ultrafast spin-polarized low energy electron microscope with multiple electron sources.

We describe the design and commissioning of a novel aberration-corrected low energy electron microscope (AC-LEEM). A third magnetic prism array (MPA) is added to the standard AC-LEEM with two prism arrays, allowing the incorporation of an ultrafast spin-polarized electron source alongside the standard cold field emission electron source, without degrading spatial resolution. The high degree of symmetries of the AC-LEEM are utilized while we design the electron optics of the ultrafast spin-polarized electron source, so as to minimize the deleterious effect of time broadening, while maintaining full control of electron spin. A spatial resolution of 2nm and temporal resolution of 10ps (ps) are expected in the future time resolved aberration-corrected spin-polarized LEEM (TR-AC-SPLEEM). The commissioning of the three-prism AC-LEEM has been successfully finished with the cold field emission source, with a spatial resolution below 2nm.

Multichannel Exchange-Scattering Spin Polarimetry.

Electron spin plays important roles in determining the physical and chemical properties of matter. However, measurements of electron spin are of poor quality, impeding the development of material sciences, because the spin polarimeter has a low efficiency. Here, we show an imaging-type exchange-scattering spin polarimeter with 6786 channels and an 8.5×10^{-3} single channel efficiency. As a demonstration, the fine spin structure of the electronic states in bismuth (111) is investigated, for which strong Rashba-type spin splitting behavior is seen in both the bulk and surface states. This improvement paves the way to study novel spin related phenomena with unprecedented accuracy.

A preliminary design of a knot undulator.

The magnetic field configuration of the previously proposed knot undulator [Qiao et al. (2009). Rev. Sci. Instrum. 80, 085108] is realised in the design of a hybridized elliptically polarized undulator, which is presented. Although the details of the field distribution are not the same as those in the theoretical proposal, it is demonstrated that the practical knot undulator could work perfectly. In order to understand the minor discrepancies of the two, mathematical formulae of the synchrotron radiation are derived based on the Fourier transform of the magnetic field. From the results of calculations by simulation program, the discrepancies could be well interpreted by the corresponding formulae. The results show the importance of optimization of the end sections of the knot undulator to suppress the on-axis heat load. Furthermore, a study of the impact of the undulator on beam dynamics of the storage ring was conducted using the Shanghai Synchrotron Radiation Facility as an example and the results show that the knot undulator has little effect on the beam.