Ultrafast Electron Diffractive Voltammetry: General Formalism and Applications

Abstract

We present a general formalism of ultrafast diffractive voltammetry approach as a contact-free tool to investigate the ultrafast surface charge dynamics in nanostructured interfaces. As case studies, the photoinduced surface charging processes in oxidized silicon surface and the hot electron dynamics in nanoparticle-decorated interface are examined based on the diffractive voltammetry framework. We identify that the charge redistribution processes appear on the surface, sub-surface, and vacuum levels when driven by intense femtosecond laser pulses. To elucidate the voltammetry contribution from different sources, we perform controlled experiments using shadow imaging techniques and N-particle simulations to aid the investigation of the photovoltage dynamics in the presence of photoemission. We show that voltammetry contribution associated with photoemission has a long decay tail and plays a more visible role in the nanosecond timescale, whereas the ultrafast voltammetry are dominated by local charge transfer, such as surface charging and molecular charge transport at nanostructured interfaces. We also discuss the general applicability of the diffractive voltammetry as an integral part of quantitative ultrafast electron diffraction methodology in researching different types of interfaces having distinctive surface diffraction and boundary conditions.

Observing charge separation in nanoantennas via ultrafast point-projection electron microscopy

“…When choosing an appropriate number of released electrons, approximately 30 per pulse, the space-time dynamics of the experimentally observed drop in differential transmission is nicely reproduced. This strongly supports that the deflection of the probe electrons quantitatively maps the expansion of the photo-released electron cloud, further substantiated by earlier works which have studied similar phenomena albeit with lower spatio-temporal resolution 29,[41][42][43] .The space-time dynamics of the electrons released from the antenna rim that is predicted by our model simulations is shown in Fig. 3d.…”

“…In principle, the observed charging may either be accounted for by photo-induced holes at the inside of the metal or by positively charged, long-lived surface states. Conceptually similar studies of photoelectron deflection by charge-separated electric fields have been performed earlier, with picosecond temporal and tens of microns spatial resolution, e.g., on cluster plasmas 40,43 , copper films surfaces or near graphite surfaces 29 . Our UPEM techniques advances the space-time resolution of such deflection studies to the 10 nm / 10 fs regime, opening up exciting avenues for probing photoinduced charge transfer and separation dynamics in individual nanostructures 35 with a time resolution that is sufficient, for instance, for probing the effects of electron-phonon interactions on those dynamics 3 .…”

Space charge effects in ultrafast electron diffraction and imaging

“…(1) considers nonlinearity in the projection imaging, the projection is in the linear regime when r y , r z ( L, d. In our front illumination geometry, the excitation laser has an elliptical footprint with r x ¼ 115 lm and r y ¼ 81 lm, determined in situ via examining surface voltammetry characterization. 19,28 Characterization of the laser footprint on the photocathode yields an initial transverse profile of the photo-emitted electrons consistent with the transverse electron bunch characterization from shadow imaging. Laser footprint characterization also yields quantitative measurements of the laser fluence (F) and, combined with R obtained from imaging, the number of emitted electrons (N e ).…”

“…8 and relevant discussions in Ref. 28. In this specific experiment, x 0 ¼ 5.0 mm and L ¼ 16.5 cm, giving a magnification M % 33 for imaging.…”

Ultrafast atomic-scale visualization of acoustic phonons generated by optically excited quantum dots

“…Second, as described in Ref. 51 in the case of a reflection geometry, the field-induced deflection is independent of the diffraction angle $\vartheta$, whereas the surface potential-induced shift of the diffraction peak decreases (in absolute value) for an increasing diffraction angle 52 (non-reciprocal behavior). Both behaviors are not consistent with the measured SWR dynamics, where the variation of the SWR resonance angle, ${\Delta \vartheta }_{SWR}$, increases (in the absolute value) for an increasing diffraction angle (reciprocal behavior) as found for the $\left(\overline{1}1\right)$, $\left(\overline{1}2\right)$, and $\left(\overline{2}2\right)$ resonances [see Fig.…”