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ABSTRACT: Direct observation and understanding of atomic-level structural dynamics are important frontiers in scientific research and applications. Femtosecond electron diffraction (FED), a technique that combines time-resolved pump-probe and electron diffraction concepts, holds a great promise to reveal the dynamical processes of ultrafast phenomena in biology, chemistry and solid-state physics at the atomic time and length scales.
This work focuses on visualizing and manipulating atomic-scale structural changes in transition metal dichalcogenide (TMD) materials, with the goals of understanding fundamental light-matter interactions in these materials and modulating material properties at an ultrafast time scale. Tungsten ditelluride is a layered TMD that crystallizes in a distorted hexagonal net with an orthorhombic unit cell (Td phase). The lack of inversion symmetry in this phase leads to a predicted new topological semimetal hosting the so-called type-II Weyl points. Here, we use a single THz pulse to trigger a structural phase transition to a centrosymmetric phase, and probe the switching using an ultrafast electron diffraction technique. These findings serve as the first direct evidence of a THz field induced structural transition in a two-dimensional material, and offer a new promising way to optically control the topological properties of solids. Additionally, we present a new non-destructive method for probing heat transport in nanoscale crystalline materials using time-resolved x-ray measurements of photo-induced strain. This technique directly probes time-dependent temperature changes in the crystal and the subsequent relaxation across buried interfaces by measuring changes in the c-axis lattice spacing after optical excitation. This work is motivated by the need to understand the fundamentals of nanoscale heat propagation, particularly at buried interfaces in functional devices.
One key property of metal-organic frameworks (MOFs) are their structural elasticity. IHere we show that 2D IR spectroscopy with pulse-shaping techniques can probe the ultrafast structural fluctuations of MOFs. 2D IR data, obtained from a vibrational probe attached to the linkers of UiO-66 MOF in low concentration, revealed that the structural fluctuations have time constants of 7 and 670 ps with no solvent. Filling the MOF pores with dimethylformamide (DMF) slows the structural fluctuations by reducing the ability of the MOF to undergo deformations, and the dynamics of the DMF molecules are also greatly restricted. Finally, methodology advances were required to remove the severe light scattering caused by the macroscopic-sized MOF particles, eliminate interfering oscillatory components from the 2D IR data, and address Förster vibrational excitation transfer.
ABSTRACT: This thesis presents the new development of the 3rd generation femtosecond diffractometer (FED) in Professor Jim Cao's group and its application to study ultrafast structural dynamics of solid state materials. The 3rd generation FED prevails its former type and other similar FED instruments by a DC electron gun that can generate much higher energy electron pulses, and a more efficient imaging system. This combination together with miscellaneous improvements significantly boosts the signal-to-noise ratio and thus enables us to study more complex solid state materials.
An experiment on structural dynamics at the ultrafast time scale in shocked metal samples is presented. The technique development of an ultrafast x-ray diffractometer to generate 'molecular movies' is described. Preliminary results of static x-ray measurements of thin unshocked Ga samples are presented. Initial experiments use 200-300 mJ of a 100fs Ti:Sapphire laser to excite K-alpha x-ray emission in an aluminum wire. The x-ray emission is relayed using a spherical crystal to the sample target. Plans for experiments using Cu K-alpha emission will also be described.
"In this thesis the design and implementation of an ultrafast electron diffractometer with radio frequency compression capabilities is presented. In addition, the results of ultrafast electron diffraction (UED) measurements on the semiconductor to metal phase transition in vanadium dioxide are shown. The ability to perform UED measurements on ultrafast time scales is first demonstrated by observing the expansion and coherent oscillation of the crystal lattice in thin film, single crystal gold. The evolution of the spatio-temporal charge density in ultrashort pulses was then studied using electron-laser cross correlation measurements mediated by the ponderomotive force. These measurements were compared with particle tracing simulations and theoretical models. Similar electron-laser cross correlation measurements were also performed in order to characterize the behaviour of a novel radio-frequency (RF) pulse compression technique. Using an RF cavity, an oscillating, 3 GHz electric field is synchronized to the electron pulse arrival time and allows for the compression of high bunch charge electron pulses (0.1 pC) to 334+/-10 fs. This represents a bunch charge increase of 10^2-10^3 over previous ultrafast electron sources that provide a sub 500 fs impulse response. Finally, the semiconductor to metal transition in vanadium dioxide was studied using RF compressed electron pulses. Here, distinct ultrafast structural and electronic phase transitions were observed providing insight into the long standing debate surrounding the roles of electron-electron interactions and electron-lattice interactions in this phase transition." --
Ultrafast Phenomena XVI presents the latest advances in ultrafast science, including both ultrafast optical technology and the study of ultrafast phenomena. It covers picosecond, femtosecond and attosecond processes relevant to applications in physics, chemistry, biology, and engineering. Ultrafast technology has a profound impact in a wide range of applications, amongst them biomedical imaging, chemical dynamics, frequency standards, material processing, and ultrahigh speed communications. This book summarizes the results presented at the 16th International Conference on Ultrafast Phenomena and provides an up-to-date view of this important and rapidly advancing field.
Advances in Imaging & Electron Physics merges two long-running serials—Advances in Electronics & Electron Physics and Advances in Optical & Electron Microscopy. The series features extended articles on the physics of electron devices (especially semiconductor devices), particle optics at high and low energies, microlithography, image science, and digital image processing, electromagnetic wave propagation, electron microscopy, and the computing methods used in all these domains. Contains contributions from leading authorities on the subject matter Informs and updates on all the latest developments in the field of imaging and electron physics Provides practitioners interested in microscopy, optics, image processing, mathematical morphology, electromagnetic fields, electron, and ion emission with a valuable resource Features extended articles on the physics of electron devices (especially semiconductor devices), particle optics at high and low energies, microlithography, image science. and digital image processing