Emerging Modalities for Condensed Matter Explorations Using Ultrafast Electron Microscopy

Over the past decade, ultrafast transmission electron microscopy (UTEM) [1] has developed immeasurably and is now a mature and widespread platform, enabling scientific explorations with nanometer-femtosecond spatiotemporal resolution, through the merge of electron microscopy and ultrafast laser technology. In essence, the UTEM is a pump-probe setup that uses femtosecond laser pulses as the pump and femtosecond electron pulses as the probe, granting us accesses to microscopic phenomena at its natural time-scale. In recent years, notable advances in ultrafast electron microscopy have utilized a specific regime of electron-light interaction, known as photon-induced near-field electron microscopy (PINEM) [2, 3]. In PINEM, the electron interacts with the electromagnetic field in the vicinity of a sample, which is excited by a narrow-band optical frequency laser excitation. This interaction imprints multiple sidebands in the electron energy spectrum, observable using electron energy loss spectroscopy (EELS) and energy-filtered TEM. This approach was employed to visualize various optical and plasmonic excitations in nano-scale systems [1-6]. One would like to harness ultrafast electron microscopy to tackle some of the longest-standing yet still-beyond-reach goals of the condensed matter community, such as light-induced superconductivity, ultrafast phase transitions, and topological phenomena in low-dimensionality and quantum materials. However, the prevalent formalisms and microscopy techniques pose several notable limitations. First, many phenomena (especially in condensed matter systems) are associated with lower frequency excitations (typically in the THz realm and below). These effects could be quite broadband and are mostly nonlinear. Second, it is not obvious what (if any) changes are induced in the electron spectrum in cases where the system is not strongly driven by a laser pulse. Other observables, contrast mechanisms, and novel microscopy modalities must be sought in order for ultrafast electron microscopy to take place in the frontier of quantum materials research. Here we present several recent realizations by our group and by other colleagues that push ultrafast electron microscopy to image novel material phenomena, using previously unperceived ways. In a recent series of publications [7-9] we propose and demonstrate an interaction mechanism enabling nanoscale imaging of the femtosecond dynamics of charge carriers. 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 (USTEM) [7]. 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 particular, this technique was employed for the study of photogenerated electron-hole distributions inside a semiconductor [7] and of free-space plasma dynamics emitted from a metal [8]. Another emerging family of techniques utilizes ultrafast electrons for imaging the near field phase (or equivalently, its sub-cycle dynamics). By recording a series of defocused energy-filtered TEM images, Gaida et al. have reconstructed the phase and amplitude of the plasmonic near-field induced on a metal tip [10]. First steps towards ultrafast off-axis electron holography in the UTEM have also been reported [11]. Departing from conventional schemes for electron phase imaging, Bucher et al. have employed two points of electron-laser PINEM-type interaction in a modified UTEM setup, thereby realizing an electron-based Ramsey interferometer [12]. In this setup, the first electron-laser interaction modulates the electron wavefunction at the laser frequency, and the second uses the modulated electron to measure the electromagnetic near-field amplitude and phase with enhanced sensitivity. Specifically, this technique was utilized to measure phonon-polariton wavepacket dynamics in hexagonal boron nitride (hBN). In related experiments, Nabben et al. [13] and Gaida et al. [14] utilized two points of electron-laser interaction to observe sub-cycle attosecond dynamics in metallic nano-tips, dielectric resonator structures, and in gold nanoprisms. These works showcase the ultrafast electron microscope as a fertile ground for the development of evermore sophisticated modalities, measuring subtle changes in electron energy and momenta with nanometer-femtosecond spatiotemporal resolution. The proposed techniques facilitate the exploration of a wide range of condensed matter phenomena, including phase transitions and charge transport effects in both classical and quantum media. Future UTEM platforms will include all of the mentioned capabilities in a single instrument: ultra-bright and coherent light-driven source with multiple electron-laser interaction points, relying on holography and 4D-STEM with sensitive spectroscopy (EELS).