The development of ultrafast electron microscopy (UEM) and variants thereof (e.g., photon-induced near-field electron microscopy, PINEM) has made it possible to image atomic-scale dynamics on the femtosecond timescale. Accessing the femtosecond regime with UEM currently relies on the generation of photoelectrons with an ultrafast laser pulse and operation in a stroboscopic pump-probe fashion. With this approach, temporal resolution is limited mainly by the durations of the pump laser pulse and probe electron packet. The ability to accurately determine the duration of the electron packets, and thus the instrument response function, is critically important for interpretation of dynamics occurring near the temporal resolution limit, in addition to quantifying the effects of the imaging mode. Here, we describe a technique for in situ characterization of ultrashort electron packets that makes use of coupling with photons in the evanescent near-field of the specimen. We show that within the weakly-interacting (i.e., low laser fluence) regime, the zero-loss peak temporal cross-section is precisely the convolution of electron packet and photon pulse profiles. Beyond this regime, we outline the effects of non-linear processes and show that temporal cross-sections of high-order peaks explicitly reveal the electron packet profile, while use of the zero-loss peak becomes increasingly unreliable.
Bibliographical noteFunding Information:
DP and DJF acknowledge support from 3M in the form of a Nontenured Faculty Award (Grant #13673369 ) and from the Donors of the American Chemical Society Petroleum Research Fund in the form of a Doctoral New Investigator Grant PRF# 53116-DNI7 . STP and AHZ acknowledge support from the National Science Foundation and the Air Force Office of Scientific Research in the Center for Physical Biology funded by the Gordon and Betty Moore Foundation.
- Instrument response function
- Photon-induced near-field electron microscopy
- Ultrafast electron microscopy