Dayne A. Plemmons

Femtosecond electron imaging of defect-modulated phonon dynamics

Precise manipulation and control of coherent lattice oscillations via nanostructuring and phonon-wave interference has the potential to significantly impact a broad array of technologies and research areas. Resolving the dynamics of individual phonons in defect-laden materials presents an enormous challenge, however, owing to the interdependent nanoscale and ultrafast spatiotemporal scales. Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge. Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering. We discover that the appearance of speed-of-sound (for example, 6 nm ps−1) wavefronts are influenced by spatially varying nanoscale strain fields, taking on the appearance of static bend contours during propagation. These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics.

Characterization of fast photoelectron packets in weak and strong laser fields in ultrafast electron microscopy.

The development of ultrafast electron microscopy (UEM) and variants thereof (e.g., photon-induced near-field electron microscopy, PINEM) has made it possible to image atomic-scale dynamics on the femtosecond timescale. Accessing the femtosecond regime with UEM currently relies on the generation of photoelectrons with an ultrafast laser pulse and operation in a stroboscopic pump-probe fashion. With this approach, temporal resolution is limited mainly by the durations of the pump laser pulse and probe electron packet. The ability to accurately determine the duration of the electron packets, and thus the instrument response function, is critically important for interpretation of dynamics occurring near the temporal resolution limit, in addition to quantifying the effects of the imaging mode. Here, we describe a technique for in situ characterization of ultrashort electron packets that makes use of coupling with photons in the evanescent near-field of the specimen. We show that within the weakly-interacting (i.e., low laser fluence) regime, the zero-loss peak temporal cross-section is precisely the convolution of electron packet and photon pulse profiles. Beyond this regime, we outline the effects of non-linear processes and show that temporal cross-sections of high-order peaks explicitly reveal the electron packet profile, while use of the zero-loss peak becomes increasingly unreliable.

Discrete Chromatic Aberrations Arising from Photoinduced Electron-Photon Interactions in Ultrafast Electron Microscopy.

In femtosecond ultrafast electron microscopy (UEM) experiments, the initial excitation period is composed of spatiotemporal overlap of the temporally commensurate pump photon pulse and probe photoelectron packet. Generation of evanescent near-fields at the nanostructure specimens produces a dispersion relation that enables coupling of the photons (ℏω = 2.4 eV, for example) and freely propagating electrons (200 keV, for example) in the near-field. Typically, this manifests as discrete peaks occurring at integer multiples (n) of the photon energy in the low-loss/gain region of electron-energy spectra (i.e., at 200 keV ± nℏω eV). Here, we examine the UEM imaging resolution implications of the strong inelastic near-field interactions between the photons employed in optical excitation and the probe photoelectrons. We find that the additional photoinduced energy dispersion occurring when swift electrons pass through intense evanescent near-fields results in a discrete chromatic aberration that limits the spatial resolving power to several angstroms during the excitation period.

Defect-mediated phonon dynamics in TaS2 and WSe2

We report correlative crystallographic and morphological studies of defect-dependent phonon dynamics in single flakes of 1T-TaS2 and 2H-WSe2 using selected-area diffraction and bright-field imaging in an ultrafast electron microscope. In both materials, we observe in-plane speed-of-sound acoustic-phonon wave trains, the dynamics of which (i.e., emergence, propagation, and interference) are strongly dependent upon discrete interfacial features (e.g., vacuum/crystal and crystal/crystal interfaces). In TaS2, we observe cross-propagating in-plane acoustic-phonon wave trains of differing frequencies that undergo coherent interference approximately 200 ps after initial emergence from distinct interfacial regions. With ultrafast bright-field imaging, the properties of the interfering wave trains are observed to correspond to the beat frequency of the individual oscillations, while intensity oscillations of Bragg spots generated from selected areas within the region of interest match well with the real-space dynamics. In WSe2, distinct acoustic-phonon dynamics are observed emanating and propagating away from structurally dissimilar morphological discontinuities (vacuum/crystal interface and crystal terrace), and results of ultrafast selected-area diffraction reveal thickness-dependent phonon frequencies. The overall observed dynamics are well-described using finite element analysis and time-dependent linear-elastic continuum mechanics.

Morphological Modulation of Acoustic Phonons Imaged with Ultrafast Electron Microscopy

Atomic-scale manipulation and control of phonon modes has been proposed and vigorously pursued for enabling and enhancing myriad technological developments [1]. Indeed, the formulation of a comprehensive microscopic description of the real-time interaction of propagating modes with individual lattice discontinuities having primary features best visualized at the atomic level (e.g., grain boundaries, step-edges, strain fields, etc.) would constitute a significant advance toward ultraprecise coherent energy manipulation and control. Fundamentally, structural dynamics of this nature are amenable to study with ultrafast methods that make use of the dependence of scattering wavevectors on lattice orientation and symmetry [2]; movement or spacing/symmetry changes of the reciprocal lattice on a fixed Ewald sphere produces a commensurate modulation or re-configuration of the resulting coherent-scattering pattern. Importantly, phase information is retained when probing dynamics in real space, thus enabling spatiotemporal localization of discrete phonon-nucleation events and resolution of propagation dynamics and frequency dispersion at morphological discontinuities.

Effects of Quantized, Transient Chromatic Aberrations in Ultrafast Electron Microscopy

High-resolution in situ electron microscopy is currently an extremely active area of research owing, in part, to the development and advancement of new instrumentation and methods for studying specimens at the atomic scale in liquids, at elevated temperatures and pressures, and under electrical biasing and during mechanical deformation. Notably, the increased experimental flexibility associated with employment of spherical aberration-correction systems – that is, relaxed requirements on accelerating voltages and pole-piece gaps – has facilitated studies of atomic-scale dynamic processes occurring under operando conditions with millisecond temporal resolution [1,2]. However, wide ranges of atomic-scale structural dynamics occur on timescales much shorter than one millisecond, the motions of which therefore cannot be resolved with current digital detector technologies.

Practical considerations for ultrashort electron pulse characterization in ultrafast transmission electron microscopy

In ultrafast transmission electron microscopy (UTEM), extension of the static analytical capabilities of transmission electron microscopy (TEM) to the ultrafast temporal domain relevant for many atomicscale processes allows for direct visualization of non-equilibrium structural phenomena [1]. Analogous to pump-probe spectroscopic techniques, atomic-scale spatiotemporal resolution is accomplished by operating a properly-modified TEM in stroboscopic mode. In this mode, an ultrafast laser pulse is divided into a pump beam used to excite the specimen in situ and a probe beam used to produce an ultrashort electron packet in the gun region; precise relative timing of the arrival of the laser pulse and electron packet at the specimen enables acquisition of information at specific time points during the dynamic response of the specimen. In this approach, temporal resolution is ultimately limited by the finite duration of the pump laser pulse and probe electron packet. Still, for dynamics occurring on a time scale comparable to the instrument response function, determination of temporal properties of the pulses in UTEM is critical for isolating the intrinsic dynamics.