Our ability to image the subnuclear region is limited not only by resolution but also by speed: the constituent particles that make up atoms and fly freely around them could, in theory, travel close to the speed of light.
In reality, electrons often move much slower, but even those slower speeds are too fast for our eyes and technology to capture. This has made it difficult to observe their behavior, but now new microscopic imaging techniques are allowing scientists to capture electrons' movements in real time.
This is the work of a team of physicists at the University of Arizona in Tucson, led by Dandang Hui and Hussein Alqattan, who can take images at speeds of attoseconds (one trillionth of a second), a technique they call atomoscopy.
“Improved time resolution in electron microscopes has been a long-awaited goal and a focus for many research groups, because we all want to see electrons moving,” says physicist Mohammed Hassan of the University of Arizona in Tucson.
“These movements happen in attoseconds. But now, for the first time, we can achieve attosecond time resolution in an electron transmission microscope, which we call 'ato-microscopy'. For the first time, we can see fragments of electrons in motion.”
Transmission Electron Microscopy (TEM) is a technique used to produce images of the smallest structures in the material world. It uses electrons instead of light to produce the images. A beam of electrons is passed through a sample of material and the interaction of the electrons with the sample produces the image. For example, below is a TEM image of a white blood cell.
TEM image of white blood cells. (Dr Jeremy Skepper/Attribution 4.0 International (CC BY 4.0))
TEM relies on the speed of the laser pulse through which the electrons are transmitted, rather than the shutter speed of a traditional camera. The faster the duration of the laser pulse, the better the image that is obtained. So if you want better image quality, the way to achieve it is to develop a laser that can fire shorter pulses.
Until now, TEM lasers have reached durations of a few attoseconds and are emitted continuously, like short bursts of static electricity.
This is a truly amazing, Nobel Prize-worthy achievement, but the problem is that although this produces a series of images, the electrons are moving a bit faster, so the changes in the electrons between the pulses are lost.
The researchers wanted to see if they could find a way to shorten the duration of the pulsed beam to just one attosecond, the speed at which electrons travel in the beam, and capture still images of them in a TEM.
A computed snapshot of electron motion in real space obtained using atto-microscopy. (Hui & Alqattan et al., Sci. Adv., 2024)
This breakthrough was achieved by splitting the pulse into three: two light pulses and one electron pulse. The first light pulse, called the pump pulse, injects energy into the graphene sample, causing the electrons to vibrate.
This is followed by a second light pulse, the gating pulse, which creates a gate, or window, and while this is “open”, a single attosecond electron pulse is shone onto the sample, capturing subatomic processes with attosecond speeds.
The result is a precise map of electron dynamics, opening the door to new research into the behavior of these important particles.
Illustration of an atom microscope. (University of Arizona)
“This transmission electron microscope is like a very powerful camera like those in your latest smartphone, allowing us to take pictures of previously invisible things like electrons,” Hassan said.
“We hope that this microscope will enable the scientific community to understand the quantum physics behind the behavior and movement of electrons.”
This research was published in Science Advances.
