US researchers have, for the first time, modified a transmission electron microscope to achieve a temporal resolution of just a few attoseconds. This novel atto-microscope can capture some of the fastest processes in chemistry, physics, or biology with a single ultrafast snapshot. This is made possible by a clever combination of laser pulses that act simultaneously as electron generators and pulse filters.
Whether in chemical reactions, cell biology, or physics, many processes occur too quickly to be captured by our eyes or conventional microscopes and cameras. Scientists are therefore working on imaging technologies with correspondingly high temporal resolution. These include optical cameras that can even capture light pulses, quantum microscopes with femtosecond resolution, and ultrafast electron microscopes. The latter can already produce a series of electron pulses at attosecond intervals.
A Turbo for the Electron Microscope
The problem, however, is that previous attosecond electron microscopes bombard the sample with an entire series of electron pulses at once. Individual pulses lasting only a few attoseconds are not possible. “The recorded snapshots therefore produce a video that shows the average dynamics of the imaged processes,” explains Dandan Hui and his colleagues from the University of Arizona in Tucson. As a result, these microscopes are well-suited for processes that repeat.
“However, to resolve non-periodic attosecond processes, such as charge movement, electron-based phase transitions, or electron movement in solids, a single attosecond pulse must be generated in the microscope,” the researchers explain. They have now developed precisely this technique for a transmission electron microscope (TEM) for the first time.
How Laser Beams Become Electron Pulses
The basis of the novel atto-electron microscope is a laser that generates ultrashort infrared laser pulses at femtosecond intervals. This pulsed laser beam is then split multiple times. About ten percent of its light is converted into ultraviolet laser pulses via a frequency converter and directed onto a photocathode in the electron microscope. This generates electron pulses that also last several hundred femtoseconds. The electron pulses are then accelerated by magnetic lenses in the microscope and directed toward the sample.
Then, the main part of the laser beam comes into play: one part excites the sample, while the rest acts as a pulse filter for the electron beam. The laser pulses strike an aluminum grid from the side, generating a scattered light field. This interacts with the electron pulses coming from above: “The electrons couple with the linearly polarized part of the scattered field, leading to an energy exchange,” explains Hui and his team. This shortens and compresses the electron pulses.
First TEM Images with Sub-Femtosecond Resolution
The result is individual electron pulses, each lasting only a few hundred attoseconds. “This increases the temporal resolution of transmission electron microscopy to the sub-femtosecond range,” the researchers explain. As a first test of the method, they used their new attomicroscope to image electron scattering in a graphene sample. They succeeded in capturing the rapidly changing patterns of electron distribution in the TEM snapshots.
“Our work paves the way for recording electron movement in four dimensions of space and time in the future,” write Hui and his colleagues. “This opens a window into the quantum world of real systems and can answer fundamental questions in physics.” But chemical and biological reactions can also be imaged and filmed in real time with this technique. “This could elevate fields such as material synthesis, drug development, and personalized medicine to a new level,” the team adds. (Science Advances, 2024; doi: 10.1126/sciadv.adp5805)
