Ultrafast Electron Microscopy of Nanoscale Charge Dynamics in Semiconductors

Abstract

The ultrafast dynamics of charge carriers in solids plays a pivotal role in emerging optoelectronics, photonics, energy harvesting, and quantum technology applications. However, the investigation and direct visualization of such nonequilibrium phenomena remains as a long-standing challenge, owing to the nanometer-femtosecond spatiotemporal scales at which the charge carriers evolve. Here, we propose and demonstrate an interaction mechanism enabling nanoscale imaging of the femtosecond dynamics of charge carriers in solids. This imaging modality, which we name charge dynamics electron microscopy (CDEM), exploits the strong interaction of free-electron pulses with terahertz (THz) near fields produced by the moving charges in an ultrafast scanning transmission electron microscope. The measured free-electron energy at different spatiotemporal coordinates allows us to directly retrieve the THz near-field amplitude and phase, from which we reconstruct movies of the generated charges by comparison to microscopic theory. The CDEM technique thus allows us to investigate previously inaccessible spatiotemporal regimes of charge dynamics in solids, providing insight into the photo-Dember effect and showing oscillations of photogenerated electron–hole distributions inside a semiconductor. Our work facilitates the exploration of a wide range of previously inaccessible charge-transport phenomena in condensed matter using ultrafast electron microscopy.

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.…”

“…The classical energy change represented by eqn (20) is a good approximation even when considering electrons as quantum wavepackets, as shown in ref. 23 and 24 . The e-beam energy variation rates can equally be separated into the corresponding contributions as .…”

“…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.…”

“…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.…”

Capturing the non-equilibrium state in light–matter–free-electron interactions through ultrafast transmission electron microscopy

“…For example, the evolution of electron clouds generated by high-flux pump lasers illuminating metallic structures and semiconductors has been studied in space and time, considering the interaction of free electrons with the terahertz near fields generated by a moving charge. [108,109] In addition to the SPP and LSPR, PINEM is expected to characterize other quasiparticles. So far, the wave packet dynamics of phonon-polariton excitations in two-dimensional materials have been studied with a midinfrared laser pump.…”