Arizona researchers develop microscope that captures electrons in motion

21 Aug 2024

Prof. Mohammed Hassan’s team describes new capability as “attomicroscopy”.

Researchers at the University of Arizona have developed the world's fastest electron microscope that can capture the movement of electrons. They believe their work will lead to groundbreaking advancements in physics, chemistry, bioengineering, materials sciences and more.

“When you get the latest version of a smart phone, it comes with a better camera,” said Mohammed Hassan, associate professor of physics and optical sciences. “Our new transmission electron microscope (TEM) is like a very powerful camera in the latest version of smart phones; it allows us to take pictures of things we were not able to see before – like electrons.”

Prof. Hassan led a team of researchers in physics and optical sciences that published the research article Attosecond electron microscopy and diffraction in the journal Science Advances.

Prof. Hassan worked alongside Nikolay Golubev, assistant professor of physics; Dandan Hui, co-lead author and former research associate in optics and physics who now works at the Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences; Husain Alqattan, co-lead author, Arizona alumnus and assistant professor of physics at Kuwait University; and graduate student Mohamed Sennary.

A TEM directs beams of electrons through the target sample. The interaction between the electrons and the sample is captured by lenses and detected by a camera sensor in order to generate detailed images.

Ultrafast electron microscopes using these principles were first developed in the 2000s and use a laser to generate pulsed beams of electrons. This technique greatly increases a microscope’s temporal resolution – its ability to measure and observe changes in a sample over time. In these ultrafast microscopes, instead of relying on the speed of a camera's shutter to dictate image quality, the resolution of a transmission electron microscope is determined by the duration of electron pulses.

Faster pulse means better image

Ultrafast electron microscopes previously operated by emitting a train of electron pulses at speeds of a few attoseconds. Pulses at these speeds create a series of images – like frames in a movie – but scientists were still missing the reactions and changes in an electron that take place in between those frames.

In order to see an electron “frozen in place”, the Arizona researchers, for the first time, generated a single attosecond electron pulse, which is as fast as electrons move, thereby enhancing the microscope’s temporal resolution, like a high-speed camera capturing movements that would otherwise be invisible.

Prof. Hassan and his colleagues based their work on the Nobel Prize-winning accomplishments of Pierre Agostini, Ferenc Krausz and Anne L’Huilliere, who won the Novel Prize in Physics in 2023 after generating the first extreme ultraviolet radiation pulse so short it could be measured in attoseconds.

Using that work as a stepping stone, the Arizona researchers developed a microscope in which a powerful laser is split and converted into two parts – a very fast electron pulse and two ultrashort light pulses.

The first light pulse, known as the pump pulse, feeds energy into a sample and causes electrons to move or undergo other rapid changes. The second light pulse, also called the optical gating pulse, acts like a gate by creating a brief window of time in which the gated, single attosecond electron pulse is generated.

The speed of the gating pulse therefore dictates the resolution of the image. By carefully synchronizing the two pulses, researchers control when the electron pulses probe the sample to observe ultrafast processes at the atomic level.

“The improvement of the temporal resolution inside of electron microscopes has been long anticipated and the focus of many research groups, because we all want to see the electron motion," Hassan said. "These movements happen in attoseconds. But now, for the first time, we are able to attain attosecond temporal resolution with our electron transmission microscope – and we have coined it ‘attomicroscopy’. For the first time, we can see pieces of the electron in motion.”