"When you get the latest version of a smartphone, it comes with a better camera," explained Mohammed Hassan, associate professor of physics and optical sciences. "This transmission electron microscope is like a very powerful camera in the latest version of smartphones; it allows us to take pictures of things we were not able to see before - like electrons. With this microscope, we hope the scientific community can understand the quantum physics behind how an electron behaves and how an electron moves."
Hassan led the team of researchers who published their findings in the journal Science Advances, under the title "Attosecond electron microscopy and diffraction." The team included Nikolay Golubev, assistant professor of physics; Dandan Hui, co-lead author and former research associate now at the Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences; Husain Alqattan, co-lead author and assistant professor of physics at Kuwait University; and Mohamed Sennary, a graduate student in optics and physics.
A transmission electron microscope magnifies objects up to millions of times their actual size, revealing details too small for traditional light microscopes. Unlike visible light microscopes, it uses beams of electrons to probe samples. The interaction between the electrons and the sample is captured by lenses and detected by a camera sensor to generate detailed images.
Ultrafast electron microscopes, first developed in the 2000s, employ lasers to create pulsed beams of electrons, significantly improving a microscope's temporal resolution-the ability to measure and observe changes in a sample over time. In these microscopes, the resolution is determined by the duration of electron pulses rather than the speed of a camera's shutter.
Previously, ultrafast electron microscopes could emit electron pulses at speeds of a few attoseconds (one quintillionth of a second). These pulses allowed scientists to create a series of images, much like frames in a movie. However, the reactions and changes in electrons that occurred between frames were missed. The U of A team has now, for the first time, generated a single attosecond electron pulse, enhancing the microscope's temporal resolution to the point where it can capture electrons frozen in motion, akin to a high-speed camera.
The researchers built on the work of Pierre Agostini, Ferenc Krausz, and Anne L'Huilliere, who won the Nobel Prize in Physics in 2023 for generating the first extreme ultraviolet radiation pulse short enough to be measured in attoseconds. Using this as a foundation, the U of A team developed a microscope in which a powerful laser is split and converted into two parts-a very fast electron pulse and two ultra-short light pulses. The first light pulse, known as the pump pulse, energizes the sample, causing electrons to move. The second pulse, called the "optical gating pulse," acts as a gate that creates a brief window of time in which the gated, single attosecond electron pulse is generated. The speed of the gating pulse determines the resolution of the image. By precisely synchronizing the two pulses, the researchers can 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 stated. "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 coined it 'attomicroscopy.' For the first time, we can see pieces of the electron in motion."
Research Report:Attosecond electron microscopy and diffraction
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