Brian Abbey

Ultrafast three-dimensional imaging of lattice dynamics in individual gold nanocrystals.

Distorted Nanoparticle Nanoparticles have found many applications in modern technology; however, the full characterization of individual particles is challenging. One of the most interesting mechanical properties is the particles response to lattice distortion. This property has been probed for ensembles of nanoparticles, but the required averaging may distort the results. Clark et al. (p. 56, published online 23 May; see the Perspective by Hartland and Lo) were able to image the generation and subsequent evolution of coherent acoustic phonons from an individual perturbed gold nanocrystal on the picosecond time scale. An x-ray free-electron laser is used to probe the elastic modes of a gold nanocrystal. [Also see Perspective by Hartland and Lo] Key insights into the behavior of materials can be gained by observing their structure as they undergo lattice distortion. Laser pulses on the femtosecond time scale can be used to induce disorder in a “pump-probe” experiment with the ensuing transients being probed stroboscopically with femtosecond pulses of visible light, x-rays, or electrons. Here we report three-dimensional imaging of the generation and subsequent evolution of coherent acoustic phonons on the picosecond time scale within a single gold nanocrystal by means of an x-ray free-electron laser, providing insights into the physics of this phenomenon. Our results allow comparison and confirmation of predictive models based on continuum elasticity theory and molecular dynamics simulations.

The soft x-ray instrument for materials studies at the linac coherent light source x-ray free-electron laser.

The soft x-ray materials science instrument is the second operational beamline at the linac coherent light source x-ray free electron laser. The instrument operates with a photon energy range of 480-2000 eV and features a grating monochromator as well as bendable refocusing mirrors. A broad range of experimental stations may be installed to study diverse scientific topics such as: ultrafast chemistry, surface science, highly correlated electron systems, matter under extreme conditions, and laboratory astrophysics. Preliminary commissioning results are presented including the first soft x-ray single-shot energy spectrum from a free electron laser.

Temporal cross-correlation of x-ray free electron and optical lasers using soft x-ray pulse induced transient reflectivity.

The recent development of x-ray free electron lasers providing coherent, femtosecond-long pulses of high brilliance and variable energy opens new areas of scientific research in a variety of disciplines such as physics, chemistry, and biology. Pump-probe experimental techniques which observe the temporal evolution of systems after optical or x-ray pulse excitation are one of the main experimental schemes currently in use for ultrafast studies. The key challenge in these experiments is to reliably achieve temporal and spatial overlap of the x-ray and optical pulses. Here we present measurements of the x-ray pulse induced transient change of optical reflectivity from a variety of materials covering the soft x-ray photon energy range from 500eV to 2000eV and outline the use of this technique to establish and characterize temporal synchronization of the optical-laser and FEL x-ray pulses.

Competition between strain-induced and temperature-controlled nucleation of InAs/GaAs quantum dots

Atomic force microscopy and photoluminescence spectroscopy (PL) have been used to study asymmetric bilayer InAs quantum dot (QD) structures grown by molecular-beam epitaxy on GaAs(001) substrates. The two QD layers were separated by a GaAs spacer layer (SL) of varying thickness and were grown at different substrate temperatures. Grown independently, these two layers would exhibit a widely different QD number density, and this technique therefore enables us to assess the influence of the strain fields created by the dots in the first layer on the second-layer QD nucleation and characteristics. For very large SLs (>40 nm), total strain relief causes the QD nucleation to be controlled exclusively by the substrate temperature, which influences the migration of In adatoms. In this case, the optical and morphological properties of the second QD layer are identical to a structure with a single QD layer grown at the same temperature. In structures with a much smaller SL, strain effects dominate over the effect of...

High-Resolution X-Ray Imaging of Plasmodium falciparum-Infected Red Blood Cells

Methods for imaging cellular architecture and ultimately macromolecular complexes and individual proteins, within a cellular environment, are an important goal for cell and molecular biology. Coherent diffractive imaging (CDI) is a method of lensless imaging that can be applied to any individual finite object. A diffraction pattern from a single biological structure is recorded and an iterative Fourier transform between real space and reciprocal space is used to reconstruct information about the architecture of the sample to high resolution. As a test system for cellular imaging, we have applied CDI to an important human pathogen, the malaria parasite, Plasmodium falciparum. We have employed a novel CDI approach, known as Fresnel CDI, which uses illumination with a curved incident wavefront, to image red blood cells infected with malaria parasites. We have examined the intrinsic X‐ray absorption contrast of these cells and compared them with cells contrasted with heavy metal stains or immunogold labeling. We compare CDI images with data obtained from the same cells using scanning electron microscopy, light microscopy, and scanning X‐ray fluorescence microscopy. We show that CDI can offer new information both within and at the surface of complex biological specimens at a spatial resolution of better than 40 nm. and we demonstrate an imaging modality that conveniently combines scanning X‐ray fluorescence microscopy with CDI. The data provide independent confirmation of the validity of the coherent diffractive image and demonstrate that CDI offers the potential to become an important and reliable new high‐resolution imaging modality for cell biology. CDI can detect features at high resolution within unsectioned cells.

On-chip photonic Fourier transform with surface plasmon polaritons

The Fourier transform (FT), a cornerstone of optical processing, enables rapid evaluation of fundamental mathematical operations, such as derivatives and integrals. Conventionally, a converging lens performs an optical FT in free space when light passes through it. The speed of the transformation is limited by the thickness and the focal length of the lens. By using the wave nature of surface plasmon polaritons (SPPs), here we demonstrate that the FT can be implemented in a planar configuration with a minimal propagation distance of around 10 μm, resulting in an increase of speed by four to five orders of magnitude. The photonic FT was tested by synthesizing intricate SPP waves with their Fourier components. The reduced dimensionality in the minuscule device allows the future development of an ultrafast on-chip photonic information processing platform for large-scale optical computing.

Whole-cell phase contrast imaging at the nanoscale using Fresnel Coherent Diffractive Imaging Tomography

X-ray tomography can provide structural information of whole cells in close to their native state. Radiation-induced damage, however, imposes a practical limit to image resolution, and as such, a choice between damage, image contrast, and image resolution must be made. New coherent diffractive imaging techniques, such Fresnel Coherent Diffractive Imaging (FCDI), allows quantitative phase information with exceptional dose efficiency, high contrast, and nano-scale resolution. Here we present three-dimensional quantitative images of a whole eukaryotic cell by FCDI at a spatial resolution below 70 nm with sufficient phase contrast to distinguish major cellular components. From our data, we estimate that the minimum dose required for a similar resolution is close to that predicted by the Rose criterion, considerably below accepted estimates of the maximum dose a frozen-hydrated cell can tolerate. Based on the dose efficiency, contrast, and resolution achieved, we expect this technique will find immediate applications in tomographic cellular characterisation.

Quantitative coherent diffractive imaging of an integrated circuit at a spatial resolution of 20 nm

The complex transmission function of an integrated circuit is reconstructed at 20 nm spatial resolution using coherent diffractive imaging. A quantitative map is made of the exit surface wave emerging from void defects within the circuit interconnect. Assuming a known index of refraction for the substrate allows the volume of these voids to be estimated from the phase retardation in this region. Sample scanning and tomography of extended objects using coherent diffractive imaging is demonstrated.

Nanoscale Fresnel coherent diffraction imaging tomography using ptychography.

We demonstrate Fresnel Coherent Diffractive Imaging (FCDI) tomography in the X-ray regime. The method uses an incident X-ray illumination with known curvature in combination with ptychography to overcome existing problems in diffraction imaging. The resulting tomographic reconstruction represents a 3D map of the specimens complex refractive index at nano-scale resolution. We use this technique to image a lithographically fabricated glass capillary, in which features down to 70nm are clearly resolved.

3D lattice distortions and defect structures in ion-implanted nano-crystals

Focussed Ion Beam (FIB) milling is a mainstay of nano-scale machining. By manipulating a tightly focussed beam of energetic ions, often gallium (Ga+), FIB can sculpt nanostructures via localised sputtering. This ability to cut solid matter on the nano-scale revolutionised sample preparation across the life, earth and materials sciences. Despite its widespread usage, detailed understanding of the FIB-induced structural damage, intrinsic to the technique, remains elusive. Here we examine the defects caused by FIB in initially pristine objects. Using Bragg Coherent X-ray Diffraction Imaging (BCDI), we are able to spatially-resolve the full lattice strain tensor in FIB-milled gold nano-crystals. We find that every use of FIB causes large lattice distortions. Even very low ion doses, typical of FIB imaging and previously thought negligible, have a dramatic effect. Our results are consistent with a damage microstructure dominated by vacancies, highlighting the importance of free-surfaces in determining which defects are retained. At larger ion fluences, used during FIB-milling, we observe an extended dislocation network that causes stresses far beyond the bulk tensile strength of gold. These observations provide new fundamental insight into the nature of the damage created and the defects that lead to a surprisingly inhomogeneous morphology.