Speaker
Description
Understanding photoemission-driven charge dynamics at metal surfaces is important for the judicious design of electron sources and studies of ultrafast plasma formation. Following femtosecond laser excitation, transient electron clouds are formed and evolve collectively on femtosecond time scales, generating rapidly varying near-surface electric fields [1-3].
Recently, the use of free-electron pulses in ultrafast TEM [1,3] and ultrafast SEM [4] has been used as a means to probe photoemission dynamics, by measuring the inelastic or elastic scattering of the free-electrons of the photoemitted electron-cloud field. Despite these efforts, an accurate and complete spatio-temporal description of the intricate dynamics of photoemission is still missing. The challenge originates from the interplay of multi-electron Coulomb interactions on ultrafast timescales, occurring in a nanometric area near the surface. Previous approaches lacked the contrast to resolve the intricate features of this process due to the rapid decay of the charge-induced fields following electron cloud expansion.
Here, we present ultrafast 4D-STEM of photoemission dynamics, creating a spatiotemporal movie of the nano-plasma field via field-induced elastic beam deflections. We use a model photoemission system - a bulk Cu surface [1]. We further compare our approach to STEM-EELS, finding that 4D-STEM remains sensitive to photoemission dynamics at lower pump fluences and later time delays (25 ps vs. only 2 ps), persisting several micrometers from the surface and revealing the long-time charge-cloud evolution. This improved sensitivity likely reflects the fact that 4D-STEM directly probes the electric fields, whereas inelastic scattering is sensitive to the faster-decaying current density generated by the moving charges. These results suggest that ultrafast 4D-STEM can access intricate ultrafast dynamics down to individual charges and novel effects like carrier cooling.
References:
[1] Madan, I. et al. ACS Nano 17, 3657 (2023).
[2] Yannai, M. et al. ACS Nano 17, 3645 (2023).
[3] Zandi, O. et al., Nat. Commun. 11, 3001 (2020).
[4] Koutenský, P. et al., ACS Photonics 12, 4452 (2025).