Extreme Ultraviolet Reflection Spectroscopy of Lanthanides and Actinides Using a High Harmonic Generation Light Source.

Absorption spectroscopy probing transitions from shallow-core d and f orbitals in lanthanides and actinides reveals information about bonding and the electronic structure in compounds containing these elements. However, spectroscopy in this photon energy range is challenging because of the limited availability of light sources and extremely short penetration depths. In this work, we address these challenges using a tabletop extreme ultraviolet (XUV), ultrafast, laser-driven, high harmonic generation light source, which generates femtosecond pulses in the 40-140 eV range. We present reflection spectroscopy measurements at the N4,5 (i.e., predominantly 4d to 5f transitions) and O4,5 (i.e., 5d to 5f transitions) absorption edges on several lanthanide and uranium oxide crystals. We compare these results to density functional theory calculations to assign the electronic transitions and predict the spectra for other lanthanides. This work paves the way for laboratory-scale XUV absorption experiments for studying crystalline and molecular f-electron systems, with applications ranging from surface chemistry, photochemistry, and electronic or chemical structure determination to nuclear forensics.

Mapping nanocrystal orientations via scanning Laue diffraction microscopy for multi-peak Bragg coherent diffraction imaging.

The recent commissioning of a movable monochromator at the 34-ID-C endstation of the Advanced Photon Source has vastly simplified the collection of Bragg coherent diffraction imaging (BCDI) data from multiple Bragg peaks of sub-micrometre scale samples. Laue patterns arising from the scattering of a polychromatic beam by arbitrarily oriented nanocrystals permit their crystal orientations to be computed, which are then used for locating and collecting several non-co-linear Bragg reflections. The volumetric six-component strain tensor is then constructed by combining the projected displacement fields that are imaged using each of the measured reflections via iterative phase retrieval algorithms. Complications arise when the sample is heterogeneous in composition and/or when multiple grains of a given lattice structure are simultaneously illuminated by the polychromatic beam. Here, a workflow is established for orienting and mapping nanocrystals on a substrate of a different material using scanning Laue diffraction microscopy. The capabilities of the developed algorithms and procedures with both synthetic and experimental data are demonstrated. The robustness is verified by comparing experimental texture maps obtained with Laue diffraction microscopy at the beamline with maps obtained from electron back-scattering diffraction measurements on the same patch of gold nanocrystals. Such tools provide reliable indexing for both isolated and densely distributed nanocrystals, which are challenging to image in three dimensions with other techniques.

Novel fabrication tools for dynamic compression targets with engineered voids using photolithography methods.

Mesoscale imperfections, such as pores and voids, can strongly modify the properties and the mechanical response of materials under extreme conditions. Tracking the material response and microstructure evolution during void collapse is crucial for understanding its performance. In particular, imperfections in the ablator materials, such as voids, can limit the efficiency of the fusion reaction and ultimately hinder ignition. To characterize how voids influence the response of materials during dynamic loading and seed hydrodynamic instabilities, we have developed a tailored fabrication procedure for designer targets with voids at specific locations. Our procedure uses SU-8 as a proxy for the ablator materials and hollow silica microspheres as a proxy for voids and pores. By using photolithography to design the targets' geometry, we demonstrate precise and highly reproducible placement of a single void within the sample, which is key for a detailed understanding of its behavior under shock compression. This fabrication technique will benefit high-repetition rate experiments at x-ray and laser facilities. Insight from shock compression experiments will provide benchmarks for the next generation of microphysics modeling.

Multi-frame, ultrafast, x-ray microscope for imaging shockwave dynamics.

Inertial confinement fusion (ICF) holds increasing promise as a potential source of abundant, clean energy, but has been impeded by defects such as micro-voids in the ablator layer of the fuel capsules. It is critical to understand how these micro-voids interact with the laser-driven shock waves that compress the fuel pellet. At the Matter in Extreme Conditions (MEC) instrument at the Linac Coherent Light Source (LCLS), we utilized an x-ray pulse train with ns separation, an x-ray microscope, and an ultrafast x-ray imaging (UXI) detector to image shock wave interactions with micro-voids. To minimize the high- and low-frequency variations of the captured images, we incorporated principal component analysis (PCA) and image alignment for flat-field correction. After applying these techniques we generated phase and attenuation maps from a 2D hydrodynamic radiation code (xRAGE), which were used to simulate XPCI images that we qualitatively compare with experimental images, providing a one-to-one comparison for benchmarking material performance. Moreover, we implement a transport-of-intensity (TIE) based method to obtain the average projected mass density (areal density) of our experimental images, yielding insight into how defect-bearing ablator materials alter microstructural feature evolution, material compression, and shock wave propagation on ICF-relevant time scales.

Understanding and correcting wavenumber error in interference pattern structured illumination imaging.

The impacts of uncertainty in mirror movements in mechanically scanned interference pattern structured illumination imaging (IPSII) are discussed. It is shown that uncertainty in IPSII mirror movements causes errors in both the phase and amplitude of the Fourier transform of the resulting imaging. Finally, we demonstrate that iterative phase retrieval algorithms can improve the quality of IPSII images by correcting the phase errors caused by mirror movement uncertainties.

Laser wavelength metrology with low-finesse etalons and Bayer filters.

We present a wavelength meter with picometer-scale resolution based on etaloning effects of inexpensive glass slides and the built-in color filters of a consumer grade CMOS camera. After calibrating the device to a commercial meter, we tested the device's calibration stability using two tunable visible lasers for a period of over 16 days. The wavelength error over that entire period has a standard deviation of 5.29 parts per million (ppm) about a most probable error of 0.90 ppm. Within 24 hours of calibration, this improves to 0.04 ppm with a standard deviation of 3.94 ppm.

Encoding the complete electric field of an ultraviolet ultrashort laser pulse in a near-infrared nonlinear-optical signal.

We introduce a variation on the cross-correlation frequency-resolved optical gating (XFROG) technique that uses a near-infrared (NIR) nonlinear-optical signal to characterize pulses in the ultraviolet (UV). Using a transient-grating XFROG beam geometry, we create a grating using two copies of the unknown UV pulse and diffract a NIR reference pulse from it. We show that, by varying the delay between the UV pulses creating the grating, the UV pulse intensity-and-phase information can be encoded into a NIR signal. We also implemented a modified generalized-projections phase-retrieval algorithm for retrieving the UV pulses from these spectrograms. We performed proof-of-principle measurements of chirped pulses and double pulses, all at 400 nm. This approach should be extendable deeper into the UV and potentially even into the extreme UV or x-ray range.

Combining Laue diffraction with Bragg coherent diffraction imaging at 34-ID-C.

Measurement modalities in Bragg coherent diffraction imaging (BCDI) rely on finding a signal from a single nanoscale crystal object which satisfies the Bragg condition among a large number of arbitrarily oriented nanocrystals. However, even when the signal from a single Bragg reflection with (hkl) Miller indices is found, the crystallographic axes on the retrieved three-dimensional (3D) image of the crystal remain unknown, and thus localizing in reciprocal space other Bragg reflections becomes time-consuming or requires good knowledge of the orientation of the crystal. Here, the commissioning of a movable double-bounce Si (111) monochromator at the 34-ID-C endstation of the Advanced Photon Source is reported, which aims at delivering multi-reflection BCDI as a standard tool in a single beamline instrument. The new instrument enables, through rapid switching from monochromatic to broadband (pink) beam, the use of Laue diffraction to determine crystal orientation. With a proper orientation matrix determined for the lattice, one can measure coherent diffraction patterns near multiple Bragg peaks, thus providing sufficient information to image the full strain tensor in 3D. The design, concept of operation, the developed procedures for indexing Laue patterns, and automated measuring of Bragg coherent diffraction data from multiple reflections of the same nanocrystal are discussed.

Cinema:Bandit: a visualization application for beamline science demonstrated on XFEL shock physics experiments.

A new visualization tool, Cinema:Bandit, and its demonstration with a continuous workflow for analyzing shock physics experiments and visually exploring the data in real time at X-ray light sources is presented. Cinema:Bandit is an open-source, web-based visualization application in which the experimenter may explore an aggregated dataset to inform real-time beamline decisions and enable post hoc data analysis. The tool integrates with experimental workflows that process raw detector data into a simple database format, and it allows visualization of disparate data types, including experimental parameters, line graphs, and images. Use of parallel coordinates accommodates the irregular sampling of experimental parameters and allows for display and filtering of both experimental inputs and measurements. The tool is demonstrated on a dataset of shock-compressed titanium collected at the Matter in Extreme Conditions hutch at the Linac Coherent Light Source.

Three-dimensional X-ray diffraction imaging of dislocations in polycrystalline metals under tensile loading.

The nucleation and propagation of dislocations is an ubiquitous process that accompanies the plastic deformation of materials. Consequently, following the first visualization of dislocations over 50 years ago with the advent of the first transmission electron microscopes, significant effort has been invested in tailoring material response through defect engineering and control. To accomplish this more effectively, the ability to identify and characterize defect structure and strain following external stimulus is vital. Here, using X-ray Bragg coherent diffraction imaging, we describe the first direct 3D X-ray imaging of the strain field surrounding a line defect within a grain of free-standing nanocrystalline material following tensile loading. By integrating the observed 3D structure into an atomistic model, we show that the measured strain field corresponds to a screw dislocation.

Carrier multiplication in semiconductor nanocrystals: influence of size, shape, and composition.

During carrier multiplication (CM), also known as multiexciton generation (MEG), absorption of a single photon produces multiple electron-hole pairs, or excitons. This process can appreciably increase the efficiency of photoconversion, which is especially beneficial in photocatalysis and photovoltaics. This Account reviews recent progress in understanding the CM process in semiconductor nanocrystals (NCs), motivated by the challenge researchers face to quickly identify candidate nanomaterials with enhanced CM. We present a possible solution to this problem by showing that, using measured biexciton Auger lifetimes and intraband relaxation rates as surrogates for, respectively, CM time constants and non-CM energy-loss rates, we can predict relative changes in CM yields as a function of composition. Indeed, by studying PbS, PbSe, and PbTe NCs of a variety of sizes we determine that the significant difference in CM yields for these compounds comes from the dissimilarities in their non-CM relaxation channels, i.e., the processes that compete with CM. This finding is likely general, as previous observations of a material-independent, "universal" volume-scaling of Auger lifetimes suggest that the timescale of the CM process itself is only weakly affected by NC composition. We further explore the role of nanostructure shape in the CM process. We observe that a moderate elongation (aspect ratio of 6-7) of PbSe NCs can cause up to an approximately two-fold increase in the multiexciton yield compared to spherical nanoparticles. The increased Auger lifetimes and improved charge transport properties generally associated with elongated nanostructures suggest that lead chalcogenide nanorods are a promising system for testing CM concepts in practical photovoltaics. Historically, experimental considerations have been an important factor influencing CM studies. To this end, we discuss the role of NC photocharging in CM measurements. Photocharging can distort multiexciton dynamics, leading to erroneous estimations of the CM yield. Here, we show that in addition to distorting time-resolved CM signals, photocharging also creates spectral signatures that mimic CM. This re-emphasizes the importance of a careful analysis of the potential effect of charged species in both optical and photocurrent-based measurements of this process.

Aspect ratio dependence of auger recombination and carrier multiplication in PbSe nanorods.

Nanomaterials with efficient carrier multiplication (CM), that is, generation of multiple electron-hole pairs by single photons, have been the object of intense scientific interest as potential enablers of high efficiency generation-III photovoltaics. In this work, we explore nanocrystal shape control as a means for enhancing CM. Specifically, we investigate the influence of aspect ratio (ρ) of PbSe nanorods (NRs) on both CM and the inverse of this process, Auger recombination. We observe that Auger lifetimes in NRs increase with increasing particle volume and for a fixed cross-sectional size follow a linear dependence on the NR length. For a given band gap energy, the CM efficiency in NRs shows a significant dependence on aspect ratio and exhibits a maximum at ρ ∼ 6-7 for which the multiexciton yields are a factor of ca. 2 higher than those in quantum dots with a similar bandgap energy. To rationalize our experimental observations, we analyze the influence of dimensionality on both CM and non-CM energy-loss mechanisms and offer possible explanations for the seemingly divergent effects the transition from zero- to one-dimensional confinement has on the closely related processes of Auger recombination and CM.

Multiexciton dynamics in infrared-emitting colloidal nanostructures probed by a superconducting nanowire single-photon detector.

Carrier multiplication (CM) is the process in which absorption of a single photon produces multiple electron-hole pairs. Here, we evaluate the effect of particle shape on CM efficiency by conducting a comparative study of spherical nanocrystal quantum dots (NQDs) and elongated nanorods (NRs) of PbSe using a time-resolved technique that is based on photon counting in the infrared using a superconducting nanowire single-photon photodetector (SNSPD). Due to its high sensitivity and low noise levels, this technique allows for accurate determination of CM yields, even with the small excitation intensities required for quantitative measurements, and the fairly low emission quantum yields of elongated NR samples. Our measurements indicate an up to ∼60% increase in multiexciton yields in NRs versus NQDs, which is attributed primarily to a decrease in the electron-hole pair creation energy. These findings suggest that shape control is a promising approach for enhancing the CM process. Further, our work demonstrates the effectiveness of the SNSPD technique for the rapid screening of CM performance in infrared nanomaterials.

Three-dimensional structure determination from a single view.

The ability to determine the structure of matter in three dimensions has profoundly advanced our understanding of nature. Traditionally, the most widely used schemes for three-dimensional (3D) structure determination of an object are implemented by acquiring multiple measurements over various sample orientations, as in the case of crystallography and tomography, or by scanning a series of thin sections through the sample, as in confocal microscopy. Here we present a 3D imaging modality, termed ankylography (derived from the Greek words ankylos meaning 'curved' and graphein meaning 'writing'), which under certain circumstances enables complete 3D structure determination from a single exposure using a monochromatic incident beam. We demonstrate that when the diffraction pattern of a finite object is sampled at a sufficiently fine scale on the Ewald sphere, the 3D structure of the object is in principle determined by the 2D spherical pattern. We confirm the theoretical analysis by performing 3D numerical reconstructions of a sodium silicate glass structure at 2 A resolution, and a single poliovirus at 2-3 nm resolution, from 2D spherical diffraction patterns alone. Using diffraction data from a soft X-ray laser, we also provide a preliminary demonstration that ankylography is experimentally feasible by obtaining a 3D image of a test object from a single 2D diffraction pattern. With further development, this approach of obtaining complete 3D structure information from a single view could find broad applications in the physical and life sciences.

Quasi-phase matching and characterization of high-order harmonic generation in hollow waveguides using counterpropagating light.

We review recent experimental and theoretical work on the use of counterpropagating light to enhance high-order harmonic generation through all-optical quasi-phase matching. Also presented is a new technique for measuring the coherence of high harmonics in the nonlinear medium. This information is crucial for understanding the process of harmonic generation over extended distances, as well as for effective enhancement using quasi-phase matching techniques.

High numerical aperture tabletop soft x-ray diffraction microscopy with 70-nm resolution.

Light microscopy has greatly advanced our understanding of nature. The achievable resolution, however, is limited by optical wavelengths to approximately 200 nm. By using imaging and labeling technologies, resolutions beyond the diffraction limit can be achieved for specialized specimens with techniques such as near-field scanning optical microscopy, stimulated emission depletion microscopy, and photoactivated localization microscopy. Here, we report a versatile soft x-ray diffraction microscope with 70- to 90-nm resolution by using two different tabletop coherent soft x-ray sources-a soft x-ray laser and a high-harmonic source. We also use field curvature correction that allows high numerical aperture imaging and near-diffraction-limited resolution of 1.5lambda. A tabletop soft x-ray diffraction microscope should find broad applications in biology, nanoscience, and materials science because of its simple optical design, high resolution, large depth of field, 3D imaging capability, scalability to shorter wavelengths, and ultrafast temporal resolution.

Lensless diffractive imaging using tabletop coherent high-harmonic soft-X-ray beams.

We present the first experimental demonstration of lensless diffractive imaging using coherent soft x rays generated by a tabletop soft-x-ray source. A 29 nm high harmonic beam illuminates an object, and the subsequent diffraction is collected on an x-ray CCD camera. High dynamic range diffraction patterns are obtained by taking multiple exposures while blocking small-angle diffraction using beam blocks of varying size. These patterns reconstruct to images with 214 nm resolution. This work demonstrates a practical tabletop lensless microscope that promises to find applications in materials science, nanoscience, and biology.