Energy-filtered ultrafast electron microscopy with chirped photoelectrons

Abstract text (incl. figure legends and references)

Introduction

Ultrafast electron microscopy (UEM) enables time-resolved imaging with a spatial resolution approaching (sub)nanometer precision. The temporal resolution has been governed by the duration of the imaging electron pulse at the specimen. The energy spread of the electron pulses develops a chirp, an energy‒time correlation, resulting in the temporal broadening of the pulses because the leading electrons with higher energies accelerate and those with lower energies are retarded during propagation.

Objectives

We demonstrate energy-filtered UEM (EFUEM), which exhibits a femtosecond resolution with a conventional energy filter for use in a TEM without pulse compression. The temporal resolution is controlled by selecting chirped photoelectrons of distinct kinetic energies using the post-specimen energy filter. We demonstrate ultrafast real-space imaging using EFUEM, observing the phase-transition dynamics of VO2 to study its heterogeneous nature with different strains exerted on individual nanoparticles (NPs) in an ensemble.

Materials & methods

A proof-of-concept experiment was performed with a polycrystalline VO2 film, which exhibits insulator-to-metal transition (IMT) from a low-temperature semiconducting phase with a low-symmetry monoclinic structure, M1, to a high-temperature metallic phase with a high-symmetry rutile structure, R. Excitation using femtosecond optical pulses provides a route to the ultrafast transition from the M1 to the R phase. To demonstrate the temporal resolution approaching the time window of IMT, time-resolved micrographs were obtained by energy filtering the photoelectrons with various slit widths. By gating the detection range of electron energy, the frame time of images is controllable.

Results

The temporal resolution is determined by the slit width of the energy filter, reaching the resolution of ~700 fs, which is mainly limited to the duration of optical excitation pulses (550 fs). The concept enables resolution of ultrafast structural responses in matter beneath temporally broad photoelectron bunch without delicate instrumental modification or compromising the large number of electrons. In our approach, the temporal resolution of ultrafast imaging was additionally limited by the degree and dispersion of the chirps of probing electron pulses, the spectral jitter of the energy filter, and the duration of the optical excitation pulses to initiate structural change. If these factors are improved, the temporal resolution of ultrafast imaging may reach <100 fs, which surpasses the timescale of serial atomic displacement. Using the energy-filtered photoelectrons, the ultrafast IMTs of VO2 NPs were visualized, revealing their unique behaviors associated with nanoscale strain. A NP under a negligible strain, such as the bulk single-crystalline VO2, transforms from the initial M1 phase directly to the R phase upon photoexcitation. When a NP in the M1 phase is partially strained, its metallic M2 phase emerges for the first few picoseconds, retaining the parent lattice structure, which then subsequently undergoes the structural transition to the R phase.

Conclusion

Our approach enables electron microscopy to access the timescale of elementary nuclear motion to visualize the onset of the structural dynamics of matter at the nanoscale