Charge Dynamics Electron Microscopy: Nanoscale Imaging of Femtosecond Plasma Dynamics

Abstract

Understanding and actively controlling the spatiotemporal dynamics of nonequilibrium electron clouds is fundamental for the design of light and electron sources, highpower electronic devices, and plasma-based applications. However, electron clouds evolve in a complex collective fashion on the nanometer and femtosecond scales, producing electromagnetic screening that renders them inaccessible to existing optical probes. Here, we solve the long-standing challenge of characterizing the evolution of electron clouds generated upon irradiation of metallic structures using an ultrafast transmission electron microscope to record the charged plasma dynamics. Our approach to charge dynamics electron microscopy (CDEM) is based on the simultaneous detection of electron-beam acceleration and broadening with nanometer/femtosecond resolution. By combining experimental results with comprehensive microscopic theory, we provide a deep understanding of this highly out-of-equilibrium regime, including previously inaccessible intricate microscopic mechanisms of electron emission, screening by the metal, and collective cloud dynamics. Beyond the present specific demonstration, the here-introduced CDEM technique grants us access to a wide range of nonequilibrium electrodynamic phenomena involving the ultrafast evolution of bound and free charges on the nanoscale.

Generation and control of localized terahertz fields in photoemitted electron plasmas

“…Likewise, the electron velocity v e and the impact parameter b are additional trajectory parameters that can be varied to explore the frequency landscape, which is also strongly dependent on sample geometry and illumination conditions. As previously reported 23,24 and discussed in Sec. 5.6 in Methods, these electric fields produce a net energy variation Δ E of the probe electron, which we plot in Fig.…”

“…To illustrate this idea, we extend our theoretical formalism to incorporate the interaction with a fast probing electron, producing excellent results in comparison with experiments, as shown in recent publications for a different geometry. 23,24 For the wedge structure investigated in this work, we consider an electron passing at a distance b from the surface, with a velocity vector v e making an angle q relative to the positive x direction, as indicated by the green arrows in Fig. 4(a) for different values of q in the a/2 # q # p − a/2 range, such that only aloof electron trajectories are considered.…”

“…where A ¼ 120 A cm À2 K À2 is the Richardson constant, I abs is the absorbed power density, 39 and a 3 ∼ 5 × 10 −36 cm 6 A −3 (ref. 23) represents the likelihood of the emission. Finally, the average energy distribution of the photoemitted electrons can be calculated as…”

“…The classical energy change represented by eqn ( 20) is a good approximation even when considering electrons as quantum wavepackets, as shown in ref. 23…”

“…22 Dense electron-plasma plumes undergo a complex spatiotemporal dynamics ruled by the collective interaction among many electrons in the presence of screening by the metallic structure, thereby posing an important challenge for a comprehensive theoretical description. Nevertheless, besides their potential for application in THz technologies, the study of this phenomenon bears interest as a source of fundamental insight into the ultrafast dynamics of complex nanoscale systems, as revealed by recent experimental results obtained by employing ultrafast electron microscopes, 23,24 whereby electron beam (ebeam) pulses are made to interact with the plasma at controlled delay times relative to the laser pulses. In fact, highenergy electrons are ideal probes for ultrafast and localized phenomena such as charged plasmas [25][26][27][28][29] due to their ultra-conned nature, enabling a spatial/temporal resolution down to sub-nanometer/femtosecond scale, combined with a high sensitivity to electromagnetic interactions.…”