arXiv subjects

David J. Flannigan

Publications and source records attributed to David J. Flannigan.

4 recordsLinked to original sources

arXiv2026

Ultrafast Electron Microscopy: A Quantitative Platform for Nonequilibrium Materials Research

Macroscopic materials function is determined not merely by equilibrium structure, but by how carriers, phonons, fields, defects, interfaces, and collective order dynamically evolve after perturbation. Ultrafast electron microscopy (UEM) uniquely bridges this gap, coupling femtosecond-to-nanosecond timing with real-space, reciprocal-space, and energy-resolved contrast. Here, we review how these integrated capabilities now quantitatively map energy flow and conversion in electronic materials, decode electronic-structural coupling in quantum and correlated systems, and isolate the localized propagation of carriers, polaritons, strain, and phonons across optoelectronic and nanomechanical architectures. We emphasize the predictive design rules now emerging from direct metrology of carrier-lattice coupling, momentum-resolved phonon thermalization, defect-controlled phase transformations, and authentic operando switching. We critically assess UEM's ongoing maturation from a qualitative, proof-of-concept experiment into a rigorous, quantitative materials platform. This paradigm shift is actively driven by integrated multimodal detection, advanced energy-resolved spectroscopy, high-repetition-rate sources, multidimensional (4D and 5D) acquisition, and physically constrained forward modeling. Ultimately, the central opportunity lies in establishing a complete, causal link from initial excitation to localized energy flow, transient structure, and macroscopic device function. By prioritizing quantitative reproducibility and robust mechanistic interpretation, UEM is positioned to engineer materials in functional states that equilibrium thermodynamics simply cannot describe.

cond-mat.mtrl-sci↗

arXiv2024

Influence of Photoemission Geometry on Timing and Efficiency in 4D Ultrafast Electron Microscopy

Broader adoption of 4D ultrafast electron microscopy (UEM) for the study of chemical, materials, and quantum systems is being driven by development of new instruments as well as continuous improvement and characterization of existing technologies. Perhaps owing to the still-high barrier to entry, the full range of capabilities of laser-driven 4D UEM instruments has yet to be established, particularly when operated at extremely low beam currents (~fA). Accordingly, with an eye on beam stability, we have conducted particle tracing simulations of unconventional off-axis photoemission geometries in a UEM equipped with a thermionic-emission gun. Specifically, we have explored the impact of experimentally adjustable parameters on the time-of-flight (TOF), the collection efficiency (CE), and the temporal width of ultrashort photoelectron packets. The adjustable parameters include the Wehnelt aperture diameter (DW), the cathode set-back position (Ztip), and the position of the femtosecond laser on the Wehnelt aperture surface relative to the optic axis (Rphoto). Notable findings include significant sensitivity of TOF to DW and Ztip, as well as non-intuitive responses of CE and temporal width to varying Rphoto. As a means to improve accessibility, practical implications and recommendations are emphasized wherever possible.

physics.app-ph↗

arXiv2024

Relative Time-of-Flight Measurement in an Ultrafast Electron Microscope

Efforts to push the spatiotemporal imaging-resolution limits of femtosecond (fs) laser-driven ultrafast electron microscopes (UEMs) to the combined angstrom-fs range will benefit from stable sources capable of generating high bunch charges. Recent demonstration of unconventional off-axis photoemitting geometries are promising, but connections to the observed onset of structural dynamics are yet to be established. Here we use the in-situ photoexcitation of coherent phonons to quantify the relative time-of-flight (r-TOF) of photoelectron packets generated from the Ni Wehnelt aperture and from a Ta cathode set-back from the aperture plane. We further support the UEM experiments with particle-tracing simulations of the precise electron-gun architecture and photoemitting geometries. In this way, we measure discernable shifts in electron-packet TOF of tens of picoseconds for the two photoemitting surfaces. These shifts arise from the impact the Wehnelt-aperture off-axis orientation has on the electron-momentum distribution, which modifies both the collection efficiency and the temporal-packet distribution relative to on-axis emission. Future needs are identified; we expect this and other developments in UEM electron-gun configuration to expand the range of materials phenomena that can be directly imaged on scales commensurate with fundamental structural dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Wehnelt Photoemission in an Ultrafast Electron Microscope: Stability and Usability

We tested and compared the stability and usability of three different cathode materials and configurations in a thermionic-based ultrafast electron microscope: (1) on-axis thermionic and photoemission from a 0.1-mm diameter LaB6 source with graphite guard ring, (2) off-axis photoemission from the Ni aperture surface of the Wehnelt electrode, and (3) on-axis thermionic and photoemission from a 0.2-mm diameter polycrystalline Ta source. For each cathode type and configuration, we illustrate how the photoelectron beam-current stability is deleteriously impacted by simultaneous cooling of the source following thermionic heating. Further, we demonstrate usability via collection of parallel- and convergent-beam electron diffraction patterns and by formation of optimum probe size. We find that usability of the off-axis Ni Wehnelt-aperture photoemission is at least comparable to on-axis LaB6 thermionic emission, as well as to on-axis photoemission. However, the stability and achievable beam currents for off-axis photoemission from the Wehnelt aperture were superior to that of the other cathode types and configurations, regardless of the electron-emission mechanism. Beam-current stability for this configuration was found to be within 1% of the mean for 70 minutes (longest duration tested), and steady-state beam current was reached within the sampling-time resolution used here (~1 s) for 15 pA beam currents (i.e., 460 electrons per packet for a 200 kHz repetition rate). Repeatability and robustness of the steady-state condition was also found to be within 1% of the mean. We discuss the implications of these findings for UEM imaging and diffraction experiments, for pulsed-beam damage measurements, and for practical switching between optimum conventional TEM and UEM operation within the same instrument.

cond-mat.mtrl-sci↗