H. A. Padmore's research while affiliated with Lawrence Berkeley National Laboratory and other places

Publications (417)

Article
We have designed and fabricated a high groove density blazed grating for a Resonant Inelastic X-ray Scattering spectrometer for the new Qerlin beamline at the Advanced Light Source (ALS) synchrotron facility. The gratings were fabricated using a set of nanofabrication techniques including e-beam lithography, nanoimprint, plasma etch, and anisotropi...
Article
Full-text available
The performance of x-ray free electron lasers and ultrafast electron diffraction experiments is largely dependent on the brightness of electron sources from photoinjectors. The maximum brightness from photoinjectors at a particular accelerating gradient is limited by the mean transverse energy (MTE) of electrons emitted from photocathodes. For high...
Article
Full-text available
The new endstation at the bending magnet beamline 7.3.1 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory (LBNL), has been commissioned and is open for general user operation. The beamline is repurposed from the previous photoemission electron microscopy (PEEM) to X-ray absorption spectroscopy (XAS) with in-situ/operando cap...
Article
This paper presents a fully ab initio many-body photoemission framework that includes coherent three-body electron-photon-phonon scattering to predict the transverse momentum distributions and the mean transverse energies (MTEs) of bulk photoelectrons from single-crystal photocathodes. The need to develop such a theory stems from the lack of studie...
Article
Full-text available
We report on the measurement of the local magnetization dynamics occurring, at units of GHz, in large aspect ratio stripes lithographed from reduced damping amorphous Fe80B20 films. The stripes were submitted to local anisotropy modifications by micrometric beam synchrotron X-ray irradiation. Our results include data on the dispersion relationships...
Article
The diaboloid is a reflecting surface that converts a spherical wave to a cylindrical wave. This complex surface may find application in new Advanced Light Source bending-magnet beamlines or in other beamlines that now use toroidal optics for astigmatic focusing. Here, the numerical implementation of diaboloid mirrors is described, and the benefit...
Article
A new type of optical element that can focus a cylindrical wave to a point focus (or vice versa) is analytically described. Such waves are, for example, produced in a beamline where light is collimated in one direction and then doubly focused by a single optic. A classical example in X-ray optics is the collimated two-crystal monochromator, with to...
Article
We have developed an advanced process for blaze angle reduction of x-ray gratings for the soft, tender, and EUV spectral ranges. The process is based on planarization of an anisotropically etched Si blazed grating followed by a chemically selective plasma etch. This provides a way to adjust the blaze angle to any lower value with high accuracy. Her...
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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...
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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...
Article
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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 inte...
Article
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 ene...
Preprint
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...
Article
Full-text available
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 detecto...
Article
We report on the induction, through X-ray irradiation, of local anisotropy modifications in amorphous Fe80B20 stripes prepared by laser lithography from 15 nm thick films deposited by pulsed laser ablation. Those anisotropy modifications, taking place in a zone having a dimension of 15 μm around the X-ray beam (2 µm width) incidence area center poi...
Preprint
Full-text available
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 paper, we demonstrate a record low mean transverse energ...
Article
In this article, we report on the development of a time-of-flight based electron energy analyzer capable of measuring the 3-D momentum and energy distributions of very low energy (millielectronvolt-scale) photoemitted electrons. This analyzer is capable for measuring energy and 3-D momentum distributions of electrons with energies down to 1 meV wit...
Preprint
In this article, we report on the development of a time-of-flight based electron energy analyzer capable of measuring the 3-D momentum and energy distributions of very low energy (meV-scale) photoemitted electrons. This analyzer is capable of measuring energy and 3-D momentum distributions of electrons with energies down to 1 meV with a sub-meV ene...
Article
The operation of photoemission electron sources with wavelengths near the photoemission threshold has been shown to dramatically decrease the minimum achievable photocathode emittance, but at the cost of significantly reduced quantum efficiency (QE). In this work, we show that for femtosecond laser and electron pulses, the increase in required lase...
Article
Full-text available
The performance of free electron laser x-ray light sources, and systems for ultrafast electron diffraction and ultrafast electron microscopy, is limited by the brightness of the electron sources used. The intrinsic emittance, or equivalently, the mean transverse energy (MTE) of electrons emitted from the photocathode determines the maximum possible...
Conference Paper
The majority of photocathode materials in use in acceler- ator applications have been discovered empirically through trial and error with little guidance from material science cal- culations. Alternatively, one can envision a process which is heavily guided by computational search using latest ad- vances in density functional theory (DFT). In this...
Conference Paper
Over the last decade, alkali-antimonides have been in- vestigated as high QE cathodes in green light and more recently as ultra-low intrinsic emittance cathodes in near- threshold red wavelengths at cryogenic temperatures [1]. Nano-meter scale surface non-uniformities (physical rough- ness and chemical roughness or work function variations) are tho...
Preprint
The operation of photoemission electron sources with wavelengths near the photoemission threshold has been shown to dramatically decrease the minimum achievable photocathode emittance, but at the cost of significantly reduced quantum efficiency (QE). In this work, we show that for femtosecond laser and electron pulses, the increase in required lase...
Article
Full-text available
We have developed a method for the manufacture of x-ray diffraction gratings with arbitrarily small blaze angles. These gratings are made by a process in which a high blaze angle grating made by anisotropic etching of Si (111) is subjected to planarization and reactive ion etching. Differential etching of the planarization medium and silicon ensure...
Article
Full-text available
The COSMIC Imaging Beamline at the Advanced Light Source: a new facility for spectro-microscopy of nano-materials - Volume 24 Supplement - David A. Shapiro, Richard Celestre, Bjoern Enders, John Joseph, Harinarayan Krishnan, Matthew A. Marcus, Kasra Nowrouzi, Howard Padmore, Jungjin Park, Anthony Warwick, Young-Sang Yu
Article
Full-text available
Nanoscale Visualization of Magnetic Contrasts with Soft X-ray Spectro-Ptychography at the Advanced Light Source - Volume 24 Supplement - Young-Sang Yu, Richard Celestre, Bjoern Enders, Kasra Nowrouzi, Howard Padmore, Tony Warwick, Jong-Ryul Jeong, David A. Shapiro
Article
In this paper, we review the status of bright electron sources. High peak current (up to kA) low emittance (<1 mm rad) electron beams have been one of the critical components in the development of XFELs. The outlook for the field is even more promising as progress in the understanding of photoemission physics, development of novel photocathode mate...
Article
Full-text available
Battery function is determined by the efficiency and reversibility of the electrochemical phase transformations at solid electrodes. The microscopic tools available to study the chemical states of matter with the required spatial resolution and chemical specificity are intrinsically limited when studying complex architectures by their reliance on t...
Article
Aberration correction by an electron mirror dramatically improves the spatial resolution and transmission of photoemission electron microscopes. We will review the performance of the recently installed aberration corrector of the X-ray Photoemission Electron Microscope PEEM-3 and show a large improvement in the efficiency of the electron optics. Ha...
Article
Full-text available
K-Cs-Sb bialkali antimonide photocathodes grown by a triple-element codeposition method have been found to have excellent quantum efficiency (QE) and outstanding near-atomic surface smoothness and have been employed in the VHF gun in the Advanced Photoinjector Experiment (APEX), however, their robustness in terms of their lifetime at elevated photo...
Article
Full-text available
Advanced photoinjectors, which are critical to many next generation accelerators, open the door to new ways of material probing, both as injectors for free electron lasers and for ultra-fast electron diffraction. For these applications, the nonuniformity of the electric field near the cathode caused by surface roughness can be the dominant source o...
Article
Detailed measurements of momentum distributions of emitted electrons have allowed the investigation of the thermal limit of the transverse emittance from metal photocathodes. Furthermore, recent developments in material design and growth have resulted in photocathodes that can deliver high quantum efficiency and are sufficiently robust to use in hi...
Article
Full-text available
We investigated the growth of Mo/Si multilayers (ML) deposited using a highly collimated flux of ion-beam sputtered particles for a wide range of deposition angles. Growth of the multilayers at normal and moderately inclined deposition is dominated by surface relaxation resulting in smooth interfaces of the multilayer stack. The first signs of inte...
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A process for fabrication of ultra-precise diffraction gratings for high resolution x-ray spectroscopy was developed. A grating pattern with constant or variable line spacing (VLS) is recorded on a quartz plate by use of e-beam lithography with nanometer scale accuracy of the groove placement. The pattern is transferred to a massive grating blank b...
Article
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We present a new ptychographic x-ray microscope dedicated to soft x-ray tomography and spectromicroscopy of nano-materials at the Advanced Light Source. The microscope utilizes an ultra-stable, high performance scanning mechanism with laser interferometer feedback for sample positioning and a fast frame rate charge-coupled device detector for soft...
Article
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The Advanced Light Source has developed a compact tomographic microscope based on soft x-ray ptychography for the study of nanoscale materials [1,2]. The microscope utilizes the sample manipulator mechanism from a commercial TEM coupled with laser interferometric feedback for zone plate positioning and a fast frame rate charge-coupled device detect...
Article
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...
Article
Reduction of roughness to the nm level is critical of achieving the ultimate performance from photocathodes used in high gradient fields. The thrust of this paper is to explore the evolution of roughness during sequential growth, and to show that deposition of multilayer structures consisting of very thin reacted layers results in an nm level smoot...
Article
K2CsSb is a promising photocathode candidate to serve as an electron source in next-generation light sources such as Free Electron Lasers (FEL) and Energy Recovery Linacs (ERL). As the traditional recipe for creation of K2CsSb photocathodes typically results in a rough surface that deteriorates electron beam quality, significant effort has been mad...
Article
We demonstrate a method for obtaining increased spatial resolution and specificity in nanoscale chemi- cal composition maps through the use of full refractive reference spectra in soft x-ray spectro-microscopy. Using soft x-ray ptychography, we measure both the absorption and refraction of x-rays through pristine reference materials as a function o...
Article
In this paper, we present a 3-D one step photoemission model that can be used to calculate the quantum efficiency and momentum distributions of electrons photoemitted from ordered single crystal surfaces close to the photoemission threshold. Using Ag(111) as an example, we show that the model can not only calculate the quantum efficiency from the s...
Article
Nano-roughness limits the emittance of electron beams that can be generated by high efficiency photocathodes, such as the thermally reacted alkali antimonide thin films. However there is an urgent need for photocathodes that can produce an order of magnitude or more lower emittance than present day systems in order to increase the transverse cohere...
Conference Paper
Full-text available
A blazed diffraction grating for the EUV lithography Beamline 12.0.1 of the Advanced Light Source has been fabricated using optical direct write lithography and anisotropic wet etching technology. A variable line spacing pattern was recorded on a photoresist layer and transferred to a hard mask layer of the grating substrate by a plasma etch. Then...
Article
Full-text available
Diffraction gratings are used from micron to nanometer wavelengths as dispersing elements in optical instruments. At shorter wavelengths crystals can be used as diffracting elements, but due to the 3D nature of the interaction with light are wavelength selective rather than wavelength dispersing. There is an urgent need to extend grating technology...
Conference Paper
Full-text available
High-resolution Resonance Inelastic X-ray Scattering (RIXS) is a technique that allows us to probe the electronic excitations of complex materials with unprecedented precision. However, the RIXS process has a low cross section, compounded by the fact that the optical spectrometers used to analyze the scattered photons can only collect a small solid...
Conference Paper
Today users visit synchrotrons as sources of understanding and discovery—not as sources of just light, and not as sources of data. To achieve this, the synchrotron facilities frequently provide not just light but often the entire end station and increasingly, advanced computational facilities that can reduce terabytes of data into a form that can r...
Conference Paper
Full-text available
High-resolution Resonant Inelastic X-ray Scattering (RIXS) requires diffraction gratings with very exacting characteristics. The gratings should provide both very high dispersion and high efficiency which are conflicting requirements and extremely challenging to satisfy in the soft x-ray region for a traditional grazing incidence geometry. To achie...
Article
Bi-alkali antimonide photocathodes are one of the best known sources of electrons for high current and/or high bunch charge applications like Energy Recovery Linacs or Free Electron Lasers. Despite their high quantum efficiency in visible light and low intrinsic emittance, the surface roughness of these photocathodes prohibits their use as low emit...
Article
The heating of the electronic distribution of a copper photocathode is calculated under the two-temperature model for ultrafast laser illumination with fluences typical for use in RF photoinjectors. Using the finite temperature-extended relations for the photocathode intrinsic emittance and quantum efficiency, the time-dependent emittance growth du...
Article
Full-text available
A 2500 lines/mm Multilayer Blazed Grating (MBG) optimized for the soft x-ray wavelength range was fabricated and tested. The grating coated with a W/B4C multilayer demonstrated a record diffraction efficiency in the 2nd blazed diffraction order in the energy range from 500 to 1200 eV. Detailed investigation of the diffraction properties of the grat...
Conference Paper
Science needs are pushing the development of MHz-class repetition-rate linac-based facilities generating high-brightness electron beams. The successful technology of lower repetition-rate RF guns cannot be scaled up to MHz rates. At LBNL, we developed the VHF-Gun, a room-temperature RF gun designed for CW operation and high-brightness beam performa...
Article
Full-text available
The advent of fully coherent free-electron laser and diffraction-limited synchrotron radiation storage ring sources of x-rays is catalyzing the development of new ultrahigh accuracy metrology methods. To fully exploit these sources, metrology needs to be capable of determining the figure of an optical element with subnanometer height accuracy. The...
Article
Measurements of the intrinsic emittance and transverse momentum distributions obtained from a metal (antimony thin film) photocathode near and below the photoemission threshold are presented. Measurements show that the intrinsic emittance is limited by the lattice temperature of the cathode as the incident photon energy approaches the photoemission...
Article
Morphological details of calcium silicate hydrate (C–S–H) stemming from the hydration process of Portland cement (PC) phases are crucial for understanding the PC-based systems but are still only partially known. Here we introduce the first soft X-ray ptychographic imaging of tricalcium silicate (C3S) hydration products. The results are compared usi...
Article
Full-text available
The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptyc...
Conference Paper
Full-text available
Multialkali antimonide photocathodes have been shown to be excellent electron sources for a wide range of applications because of high quantum efficiency, low emittance, good lifetime, and fast response. In recent years, synchrotron X-ray methods have been used to study the growth mechanism of K2CsSb photocathodes. The traditional sequential growth...
Conference Paper
After the formidable results of X-ray 4th generation light sources based on free electron lasers around the world, a new revolutionary step is undergoing to extend the FEL performance from the present few hundred Hz to MHz-class repetition rates. In such facilities, temporally equi-spaced pulses will allow for a wide range of previously non-accessi...
Conference Paper
Full-text available
Alkali antimonide photocathodes are a strong contender for the cathode of choice for next-generation photon sources such as LCLS II or the XFEL. These materials have already found extensive use in photodetectors and image intensifiers. However, only recently have modern synchrotron techniques enabled a systematic study of the formation chemistry of...