Real-Space Imaging of Picosecond Structural Dynamics in Metals with Ultrafast Transmission Electron Microscopy

Abstract

Probing the first picoseconds of structural response after photoexcitation in metal systems allows us to construct a timeline of how the absorbed energy evolves in space and time. To do this, we need a technique that is sensitive to structural changes at nanometer-picosecond spatiotemporal resolutions. Often techniques that fit these requirements operate in reciprocal space (e.g., ultrafast electron and x-ray diffraction) which are spot-size limited resulting in ensemble averaging over interesting defect and microstructure interactions. Here we use ultrafast transmission electron microscopy (UEM) to image the structural dynamics of two metal materials systems, the iron pnictide LaFeAsO and plasmonic gold nanorods, after ultrafast photoexcitation. First, plasmonic gold nanorods offer a viable route for coupling light into structures smaller than its wavelength. However, there is a convolution of optical and structural effects that prevent a complete understanding of how plasmonic structures behave, specifically in assemblies. The goal of this work is to use imaging in UEM to elucidate the structural response of a non-trivial assembly of gold nanorods to serve as complimentary structural information that when combined with optical models can form a complete picture. Investigations reveal early incoherent-to-coherent structural dynamics from an increase in thermal diffuse scattering followed by a roughly 10 ps delay believed to be phonon-phonon coupling ending with the launch and propagation of strain waves at the speed of sound. Because we are imaging, the response from individual nanorods in the assembly can be resolved as well as the potential effect on neighboring rods. This work has shown that we can construct a timeline for the first 100 picoseconds after ultrafast excitation from the electron-phonon coupling to the whole nanorod acoustic modes. Second, the iron pnictide class of materials display unconventional superconductivity when doped. Mechanistically different than the cuprates, the iron pnictide superconductivity has yet to be fully understood. It is believed that the nearly simultaneous structural and magnetic phase transitions of the parent compound offer clues to understanding the superconductivity mechanism. We use imaging in UEM to explore the structural dynamics of the parent compound. After photoexcitation, hypersonic Lamb modes emanate from the crystal-vacuum interface towards the bulk then interact with microscale crystal boundaries (e.g. step-edges, interfaces) that lead to reflection and wave interference. In addition, we present some initial exploration of probing the structural phase transition; showcasing the differences between the tetragonal and orthorhombic phases while demonstrating the capability of UEM to detect changes in elastic constants. The observations from these experiments show the strength of UEM to capture real-space, structural dynamics not easily accessible with reciprocal space techniques to provide complimentary information to the materials science and physics community.