Free-Electron–THz Interactions As A Probe For Condensed Matter Explorations

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.

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, and ultrafast phase transitions. However, the prevalent formalisms and microscopy techniques pose several notable limitations, as many phenomena in condensed matter systems are associated with THz frequency excitations, and it is not obvious what (if any) changes are induced in the probe electron in such cases. 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.

In a recent series of publications^2-4 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 in the UTEM with terahertz (THz) near-fields produced by the motion of charge carriers. 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 underlaying charge dynamics by comparison to microscopic theory. In particular, this technique was employed for the study of photo-generated electron-hole distributions inside a semiconductor² and of free-space plasma dynamics emitted from a metal^3,4.

Another important implication of the electron-THz interaction is the ability to shape the probe electron phase space. This shaping can be tuned to create monochromated electron pulses⁵, thus facilitating spectroscopic measurements of low-energy condensed matter excitations.

References:

Zewail, Science 328 (2010).

Yannai et al., ACS Nano 17 (2023).

Madan et al., ACS Nano 17 (2023).

J.C. Dias et al., Nanoscale Adv. 5 (2023).

Yannai et al., Physical Review Letters 131 (2024).

PhD  under the supervision of Prof. Ido Kaminer.