The researchers used powerful laser flashes to irradiate thin films of crystalline materials, which drove crystal electrons into a fast wiggling motion. As the electrons bounced off 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 for 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 understand and eventually control the chemical and the electronic properties of materials.
Such lasers flashes can now track ultrafast microscopic processes inside solids. Still, they cannot spatially resolve electrons, i.e., see how electrons occupy the minute space among atoms in crystals, or how they form the chemical bonds that hold atoms together. Ernst Abbe discovered the reason more than a century ago. 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, the Light Picoscope.
"A powerful laser pulse can force electrons inside crystalline materials to become the photographers of the space around them," said Harshit Lakhotia, a researcher of the group.
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 Lakhotia.