Donald A. Walko

Picosecond time-resolved laser pump/X-ray probe experiments using a gated single-photon-counting area detector.

The recent developments in X-ray detectors have opened new possibilities in the area of time-resolved pump/probe X-ray experiments; this article presents the novel use of a PILATUS detector to achieve X-ray pulse duration limited time-resolution at the Advanced Photon Source (APS), USA. The capability of the gated PILATUS detector to selectively detect the signal from a given X-ray pulse in 24 bunch mode at the APS storage ring is demonstrated. A test experiment performed on polycrystalline organic thin films of alpha-perylene illustrates the possibility of reaching an X-ray pulse duration limited time-resolution of 60 ps using the gated PILATUS detector. This is the first demonstration of X-ray pulse duration limited data recorded using an area detector without the use of a mechanical chopper array at the beamline.

Relationship between planar GaAs nanowire growth direction and substrate orientation.

Planar GaAs nanowires are epitaxially grown on GaAs substrates of various orientations, via the Au-catalyzed vapor-liquid-solid mechanism using metal organic chemical vapor deposition. The nanowire geometry and growth direction are examined using scanning electron microscopy and x-ray microdiffraction. A hypothesis relating the planar nanowire growth direction to the surface projections of [111] B crystal directions is proposed. GaAs planar nanowire growth on vicinal substrates is performed to test this hypothesis. Good agreement between the experimental results and the projection model is found.

Structural and electronic recovery pathways of a photoexcited ultrathin VO 2 film

The structural and electronic recovery pathways of a photoexcited ultrathin VO2 film at nanosecond time scales have been studied using time-resolved x-ray diffraction and transient optical absorption techniques. The recovery pathways from the tetragonal metallic phase to the monoclinic insulating phase are highly dependent on the optical pump fluence. At pump fluences higher than the saturation fluence of 14.7 mJ/cm2, we observed a transient structural state with lattice parameter larger than that of the tetragonal phase, which is decoupled from the metal-to-insulator phase transition. Subsequently, the photoexcited VO2 film recovered to the ground state at characteristic times dependent upon the pump fluence, as a result of heat transport from the film to the substrate. We present a procedure to measure the time-resolved film temperature by correlating photoexcited and temperature-dependent x-ray diffraction measurements. A thermal transport model that incorporates changes of the thermal parameters across the phase transition reproduces the observed recovery dynamics. The optical excitation and fast recovery of ultrathin VO2 films provides a practical method to reversibly switch between the monoclinic insulating and tetragonal metallic state at nanosecond time scales.

Time-Resolved Research at the Advanced Photon Source Beamline 7-ID

The Sector 7 undulator beamline (7‐ID) of the Advanced Photon Source (APS) is dedicated to time‐resolved x‐ray research and is capable of ultrafast measurements on the order of 100 ps. Beamline 7‐ID has a laser laboratory featuring a Ti:Sapphire system (average power of 2.5 W, pulse duration <50 fs, repetition rate 1–5 kHz) that can be synchronized to the bunch pattern of the storage ring. The laser is deliverable to x‐ray enclosures, which contain diffractometers, as well as motorized optical tables for table‐top experiments. Beamline 7‐ID has a single APS Undulator A and uses a diamond (111) double‐crystal monochromator, providing good energy resolution over a range of 6–24 keV. Available optics include Kirkpatrick‐Baez (KB) mirrors to microfocus the x‐ray beam. A variety of time‐resolved diffraction and spectroscopy research is available at 7‐ID, with experiments being done in the atomic, molecular, optical, chemistry, and solid state (bulk and surface) fields.

Realization of Unidirectional Planar GaAs Nanowires on GaAs (110) Substrates

A self-aligned unidirectional planar GaAs nanowire (NW) array is realized by growing on (110) GaAs substrates through the Au-catalyzed vapor-liquid-solid mechanism. All NWs on (110) substrates propagate along the [00-1] direction, yielding planar NWs with trapezoidal cross sections where the top surface and sidewalls are identified by micro X-ray diffraction analysis to be [110], [010], and [100] facets, respectively. Depletion-mode long-channel metal-semiconductor field-effect transistors using these [00-1] GaAs NWs as channels exhibit well-defined dc output and transfer characteristics, confirming the high material quality of the NWs. Completely ordered site controlled arrays of planar NWs are demonstrated by growing on (110) substrates with Au catalyst nanoparticles patterned using electron beam lithography.

Empirical dead-time corrections for synchrotron sources.

An experimental comparison of models for performing dead-time corrections of photon-counting detectors at synchrotron sources is presented. The performance of several detectors in the three operating modes of the Advanced Photon Source is systematically compared, with particular emphasis on asymmetric fill patterns. Several simple and well known correction formulas are evaluated. The results demonstrate the critical importance of detector speed and synchrotron fill pattern in selecting the proper dead-time correction.

Structural investigation of CoMnGe combinatorial epitaxial thin films using microfocused synchrotron X-ray

We discuss important experimental considerations and high-throughput synchrotron-based techniques for structural characterization of binary and ternary composition-spread thin films. We apply these techniques to obtain detailed structural phase diagrams of CoMnGe ternary alloy system.

Composition characterization of combinatorial materials by scanning X-ray fluorescence microscopy using microfocused synchrotron X-ray beam

Abstract We describe a high-throughput scanning X-ray fluorescence (XRF) microscopy setup using a microfocused synchrotron X-ray beam, which is optimized for in-parallel X-ray characterization of composition and crystalline structure of combinatorial samples. We present X-ray fluorescence elemental maps of a full ternary Co x Mn y Ge 1− x − y composition-spread thin film and discuss the quantitative analysis method used for obtaining the ternary composition.

Application of X-ray fluorescence to turbulent mixing.

Combined measurements of X-ray absorption and fluorescence have been performed in jets of pure and diluted argon gas to demonstrate the feasibility of using X-ray fluorescence to study turbulent mixing. Measurements show a strong correspondence between the absorption and fluorescence measurements for high argon concentration. For lower argon concentration, fluorescence provides a much more robust measurement than absorption. The measurements agree well with the accepted behavior of turbulent jets.

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO3

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained. (Less)