Picoscope: laser takes pictures of electrons in crystals
A novel microscopy method achieves picometer resolution with excitation by visible light. It may become possible to record real movies of electrons in materials.
To date, microscopes of visible light cannot discern how electrons are distributed among atoms in solids. Now researchers around Professor 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 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.
"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. He adds, "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.”
Now the researchers are working on enhance the technique. 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.
[Lakhotia et al., Laser picoscopy of valence electrons in solids, Nature 583, 55–59 (2020). DOI: 10.1038/s41586-020-2429-z]