Laser Takes Pictures of Electrons in Crystals

The new picoscope makes pictures possible to illustrate how the electron cloud is distributed among atoms in the crystal lattice of solids with a resolution of a few tens of picometers. (Source: C. Hackenberger, U. Rostock)

Microscopes of visible light allow us to see tiny objects such living cells and their interior. Yet, they cannot discern how electrons are distributed among atoms in solids. Now researchers around Eleftherios Gouliel­makis of the Extreme Photonics Labs at the Uni­versity of Rostock and the Max Planck Institute of Quantum Optics in Garching, Germany, along with coworkers of the Institute of Physics of the Chinese Academy of Sciences in Beijing, developed a new type of a light micro­scope, the Picoscope, that allows overcoming this limi­tation.

The researchers used powerful laser flashes to irradiate thin, films of crystal­line materials. These laser pulses drove crystal electrons into a fast wiggling motion. As the electrons bounced off with the sur­rounding electrons, they emitted radiation in the extreme ultraviolet part of the spectrum. By analyzing the properties of this radiation, the researchers composed pictures that illus­trate how the electron cloud is distri­buted among atoms in the crystal lattice of solids with a resolution of a few tens of picometers. The experiments pave the way towards developing a new class of laser-based micro­scopes that could allow physicists, chemists, and material scientists to peer into the details of the microcosm with unpre­cedented resolution and to deeply understand and eventually control the chemical and the electronic properties of materials.

For decades scientists have used flashes of laser light to understand the inner workings of the microcosm. Such lasers flashes can now track ultrafast micro­scopic processes inside solids. Still they cannot spatially resolve electrons, that is, to see how electrons occupy the minute space among atoms in crystals, and how they form the chemical bonds that hold atoms together. The reason is long known. It was disco­vered by Abbe more than a century back. Visible light can only discern objects commensurable in size to its wave­length which is approximately few hundreds of nanometers. But to see electrons, the microscopes have to increase their magni­fication power by a few thousand times. To overcome this limi­tation, Goulielmakis and coworkers took a different path.

They developed a microscope that works with powerful laser pulses. They dubbed their device as the Light Picoscope. “A powerful laser pulse can force electrons inside crystal­line materials to become the photo­graphers of the space around them.” When the laser pulse penetrates inside the crystal, it can grab an electron and drive it into a fast- wiggling motion. “As the electron moves, it feels the space around it, just like your car feels the uneven surface of a bumpy road,” said Harshit Lakhotia, a researcher of the group. When the laser-driven electrons cross a bump made by other electrons or atoms, it decelerates and emits radia­tion at a frequency much higher than that of the lasers. “By recording and analyzing the properties of this radiation, we can deduce the shape of these minute bumps, and we can draw pictures that show where the electron density in the crystal is high or low,” said Hee-Yong Kim, a doctorate researcher in Extreme Photonics Labs. “Laser Pico­scopy combines the capability of peering into the bulk of materials, like x-rays, and that of probing valence electrons. The latter is possible by scanning tunneling micro­scopes but only on surfaces.”

“With a microscope capable of probing, the valence electron density we may soon be able to benchmark the perfor­mance of compu­tational solid-state physics tools,” said Sheng Meng, from the Institute of Physics, Beijing, and a theoretical solid-state physicist in the research team. “We can optimize modern, state-of-the-art models to predict the pro­perties of materials with ever finer detail. This is an exciting aspect that laser picoscopy brings in,” he continues.

Now the researchers are working on developing the technique further. They plan to probe electrons in three dimensions and further benchmark the method with a broad range of materials including 2-D and topo­logical materials. “Because laser picoscopy can be readily combined with time-resolved laser techniques, it may soon become possible to record real movies of electrons in materials. This is a long-sought goal in ultrafast sciences and micro­scopies of matter”, Goulielmakis concludes. (Source: U. Rostock)

Reference: H. Lakhotia et al.: Laser picoscopy of valence electrons in solids, Nature 583, 55 (2020); DOI: 10.1038/s41586-020-2429-z

Link: Extreme Photonics Lab, Institute of Physics, University of Rostock, Rostock, Germany

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