Photon gating in four-dimensional ultrafast electron microscopy

Abstract

Ultrafast electron microscopy (UEM) is a pivotal tool for imaging of nanoscale structural dynamics with subparticle resolution on the time scale of atomic motion. Photon-induced near-field electron microscopy (PINEM), a key UEM technique, involves the detection of electrons that have gained energy from a femtosecond optical pulse via photon-electron coupling on nanostructures. PINEM has been applied in various fields of study, from materials science to biological imaging, exploiting the unique spatial, energy, and temporal characteristics of the PINEM electrons gained by interaction with a "single" light pulse. The further potential of photon-gated PINEM electrons in probing ultrafast dynamics of matter and the optical gating of electrons by invoking a "second" optical pulse has previously been proposed and examined theoretically in our group.Here, we experimentally demonstrate this photon-gating technique, and, through diffraction, visualize the phase transition dynamics in vanadium dioxide nanoparticles. With optical gating of PINEM electrons, imaging temporal resolution was improved by a factor of 3 or better, being limited only by the optical pulse widths. This work enables the combination of the high spatial resolution of electron microscopy and the ultrafast temporal response of the optical pulses, which provides a promising approach to attain the resolution of few femtoseconds and attoseconds in UEM.photon-induced near-field electron microscopy | attosecond electron microscopy | optical-gated electron pulse | time-resolved PINEM | photon-electron gating I n ultrafast electron microscopy (UEM) (1-3), electrons generated by photoemission at the cathode of a transmission electron microscope are accelerated down the microscope column to probe the dynamic evolution of a specimen initiated by an ultrafast light pulse. The use of femtosecond lasers to generate the electron probe and excite the specimen has made it possible to achieve temporal resolution on the femtosecond time scale, as determined by the cross-correlation of the optical and electron pulses. One important method in the UEM repertoire is photon-induced near-field electron microscopy (PINEM) (4, 5), in which the dynamic response detected by the electron probe is the pump-induced charge density redistribution in nanoscale specimens (6).Photon-electron coupling is the basic building block of PINEM, which takes place in the presence of nanostructures when the energy-momentum conservation condition is satisfied (4, 5). This coupling leads to inelastic gain/loss of photon quanta by electrons in the electron packet, which can be resolved in the electron energy spectrum (5,7,8). This spectrum consists of discrete peaks, spectrally separated by multiples of the photon energy (nZω), on the higher and lower energy sides of the zero loss peak (ZLP) (4) (Fig. 1). The development of PINEM enables the visualization of the spatiotemporal dielectric response of nanostructures (9), visualization of plasmonic fields (4, 5) and their spatial interferences (10), imag...

Electron microscopy of electromagnetic waveforms

“…To have as short as possible electron pulses at the sample position, different techniques can be used, for example, laser-triggered streaking [53], single-shot time-stamping [54], energy filtering [55] or optical gating [56,57]. Compression with microwave cavities provides 150-fs (full width at half maximum) individual free electron pulses in the multi-electron regime [58] and 10-fs-pulses (root mean square) for single-electron wave packets [40].…”

Perspective: 4D ultrafast electron microscopy—Evolutions and revolutions

“…The material studied is vanadium dioxide nanoparticles which undergo insulator-to-metal phase transition when appropriately excited. 49 Adapted with permission from M. T. 88 their efficiency and functionality, and 4D EM studies of such devices 77,78 are invaluable in doing so. On the biological front, one problem that can now be tackled using the already developed cryo-4D EM [67][68][69] is folding/unfolding in proteins.…”

“…In all the previous experiments conducted in 4D UEM, a single optical pulse had been used to initiate the change in the nanostructure. In a recent report, 49 based on the conceptual framework given in Ref. 64, we utilized two optical pulses to generate excitation and one electron pulse to monitor the structural change.…”

“…These hurdles could only be overcome by the development of single-electron imaging, 48 and by exploiting "ultrashort optical pulses" in simultaneous generation of electron packets for imaging and electronic excitation (or temperature jump) initiating the dynamical change in the specimen. Gating of electron pulses 49 or compression with RF 50 can make them accommodate large numbers of electrons. In UEM, we are not limited by the microscope's CCD detector response!…”

Infrared PINEM developed by diffraction in 4D UEM

“…In addition, focused-beam PINEM has been used in scanning TEM mode to obtain induced near-field distributions for a copper grid bar (21), a nanometer gold tip (22), and a silver nanoparticle at the subparticle level (21). In a recent publication, three pulses, two optical and one electron, were introduced into the arsenal of techniques to gate the electron pulse and make its width only limited by the optical-pulse durations (23). Numerous general theoretical treatments (24-28) have successfully described the phenomenon, with detailed treatments quantitatively reproducing many unique features of these multifaceted experimental observations (17,(20)(21)(22).…”