Showing papers by "Howard A. Padmore published in 2020"
TL;DR: A novel ultrahigh-resolution x-ray microscope achieves 8-nm spatial resolution and accurately maps chemistry in nanomaterials.
Abstract: The analysis of chemical states and morphology in nanomaterials is central to many areas of science. We address this need with an ultrahigh-resolution scanning transmission soft x-ray microscope. Our instrument provides multiple analysis tools in a compact assembly and can achieve few-nanometer spatial resolution and high chemical sensitivity via x-ray ptychography and conventional scanning microscopy. A novel scanning mechanism, coupled to advanced x-ray detectors, a high-brightness x-ray source, and high-performance computing for analysis provide a revolutionary step forward in terms of imaging speed and resolution. We present x-ray microscopy with 8-nm full-period spatial resolution and use this capability in conjunction with operando sample environments and cryogenic imaging, which are now routinely available. Our multimodal approach will find wide use across many fields of science and facilitate correlative analysis of materials with other types of probes.
42 citations
TL;DR: This Letter demonstrates a record low mean transverse energy of 5 meV from the cryo-cooled surface of copper using near-threshold photoemission and shows that the electron energy spread obtained from such a surface is less than 11.5 MeV, making it the smallest energy spread electron source known to date.
Abstract: Achieving a low mean transverse energy or temperature of electrons emitted from the photocathode-based electron sources is critical to the development of next-generation and compact x-ray free electron lasers and ultrafast electron diffraction, spectroscopy, and microscopy experiments. In this Letter, we demonstrate a record low mean transverse energy of 5 meV from the cryo-cooled (100) surface of copper using near-threshold photoemission. Further, we also show that the electron energy spread obtained from such a surface is less than 11.5 meV, making it the smallest energy spread electron source known to date: more than an order of magnitude smaller than any existing photoemission, field emission, or thermionic emission based electron source. Our measurements also shed light on the physics of electron emission and show how the energy spread at few meV scale energies is limited by both the temperature and the vacuum density of states.
34 citations
TL;DR: A cantilevered liquid-nitrogen-cooled silicon mirror is described that will achieve diffraction-limited performance even under extreme power density.
Abstract: This paper presents a novel cantilevered liquid-nitrogen-cooled silicon mirror design for the first optic in a new soft X-ray beamline that is being developed as part of the Advanced Light Source Upgrade (ALS-U) (Lawrence Berkeley National Laboratory, USA). The beamline is optimized for photon energies between 400 and 1400 eV with full polarization control. Calculations indicate that, without correction, this design will achieve a Strehl ratio greater than 0.85 for the entire energy and polarization ranges of the beamline. With a correction achieved by moving the focus 7.5 mm upstream, the minimum Strehl ratio is 0.99. This design is currently the baseline plan for all new ALS-U insertion device beamlines.
9 citations
TL;DR: The optical design of a soft X-ray spectrometer that utilizes Wolter mirrors to enable imaging-mode spectroscopy with a beam-size independent spatial resolution in, for example, tandem catalysts in reaction is presented.
Abstract: The optical design of a Hettrick–Underwood-style soft X-ray spectrometer with Wolter type 1 mirrors is presented. The spectrometer with a nominal length of 3.1 m can achieve a high resolving power (resolving power higher than 10000) in the soft X-ray regime when a small source beam (<3 µm in the grating dispersion direction) and small pixel detector (5 µm effective pixel size) are used. Adding Wolter mirrors to the spectrometer before its dispersive elements can realize the spatial imaging capability, which finds applications in the spectroscopic studies of spatially dependent electronic structures in tandem catalysts, heterostructures, etc. In the pump–probe experiments where the pump beam perturbs the materials followed by the time-delayed probe beam to reveal the transient evolution of electronic structures, the imaging capability of the Wolter mirrors can offer the pixel-equivalent femtosecond time delay between the pump and probe beams when their wavefronts are not collinear. In combination with some special sample handing systems, such as liquid jets and droplets, the imaging capability can also be used to study the time-dependent electronic structure of chemical transformation spanning multiple time domains from microseconds to nanoseconds. The proposed Wolter mirrors can also be adopted to the existing soft X-ray spectrometers that use the Hettrick–Underwood optical scheme, expanding their capabilities in materials research.
9 citations
TL;DR: A realistic wave optics simulation method has been developed to study how wavefront distortions originating from heat load deformations can be corrected using adaptive X-ray optics.
Abstract: A realistic wave optics simulation method has been developed to study how wavefront distortions originating from heat load deformations can be corrected using adaptive X-ray optics. Several planned soft X-ray and tender X-ray insertion-device beamlines in the Advanced Light Source upgrade rely on a common design principle. A flat, first mirror intercepts the white beam; vertical focusing is provided by a variable-line-space monochromator; and horizontal focusing comes from a single, pre-figured, adaptive mirror. A variety of scenarios to cope with thermal distortion in the first mirror are studied by finite-element analysis. The degradation of the intensity distribution at the focal plane is analyzed and the adaptive optics that correct it is modeled. The range of correctable wavefront errors across the operating range of the beamlines is reported in terms of mirror curvature and spatial frequencies. The software developed is a one-dimensional wavefront propagation package made available in the OASYS suite, an adaptable, customizable and efficient beamline modeling platform.
8 citations
21 Aug 2020
TL;DR: In this paper, the authors proposed a planarization of saw-tooth surfaces of blazed gratings used for the monochromatization of light, but the applications should be much wider.
Abstract: Planarization is important in many areas of nanostructure fabrication. Here we describe a new process for planarization saw-tooth surface of blazed gratings used for the monochromatization of light, but the applications should be much wider. Such gratings consist of relatively wide and very shallow triangular grooves with slanted facets which are machined with nanometer accuracy. The process of making such gratings includes planarization of a relatively coarse saw-tooth surface with micron deep grooves following by a plasma etch which provides reduction of the facet angle and hence groove depth by a factor of 10 - 100. To achieve high quality of the final grating the planarization step should provide a flat surface over the grating facets with sub-nanometer level planarity. We investigated planarization of coarse saw-tooth surfaces with a groove width of 10 μm and a facet angle of 4° by a polymer coating spun on the grating. The optimized planarization procedure provides 100% planarization even on these highly structured surfaces.
5 citations
21 Aug 2020
TL;DR: In this article, a cantilevered-liquid-nitrogen-cooled-silicon mirror for a new insertion device beamline included in the Advanced Light Source Upgrade (ALS-U) is presented.
Abstract: In this paper we provide an update on the development of a novel cantilevered-liquid-nitrogen-cooled-silicon mirror for a new insertion device beamline included in the Advanced Light Source Upgrade (ALS-U). The goals of this mirror development are to achieve diffraction limited performance, demonstrate reliability, minimize coolant flow induced vibration, and demonstrate carbon contamination prevention and cleaning techniques. In this paper we summarize the design requirements, the design of the mirror system, and prototype fabrication.
Posted Content•
TL;DR: In this article, the transverse momentum distribution and mean transverse energies of photoelectrons from single-crystal photocathodes were predicted and the results showed that photoexcitations into bulk-like states and coherent, many-body electron-photon-phonon scattering processes played important roles in photoemission from PbTe(111).
Abstract: This manuscript presents, to our knowledge, the first fully ab initio many-body photoemission framework to predict the transverse momentum distributions and the mean transverse energies (MTEs) of photoelectrons from single-crystal photocathodes. The need to develop such a theory stems from the lack of studies that provide complete understanding of the underlying fundamental processes governing the transverse momentum distribution of photoelectrons emitted from single crystals. For example, initial predictions based on density-functional theory calculations of effective electron masses suggested that the (111) surface of PbTe would produce very small MTEs ($\leq$ 15 meV), whereas our experiments yielded MTEs ten to twenty times larger than these predictions, and also exhibited a lower photoemission threshold than predicted. The ab initio framework presented in this manuscript correctly reproduces the magnitude of the MTEs from our measurements in PbTe(111) and also the observed photoemission below the predicted threshold. Our results show that photoexcitations into bulk-like states and coherent, many-body electron-photon-phonon scattering processes, both of which initial predictions ignored, indeed play important roles in photoemission from PbTe(111). Finally, from the lessons learned, we recommend a procedure for rapid computational screening of potential single-crystal photocathodes for applications in next-generation ultrafast electron diffraction and X-ray free-electron lasers, which will enable new, significant advances in condensed matter research.