Exploring quantum electron highways with laser light

Topological insulators, or TIs, have two faces: Electrons flow freely along their surface edges, like cars on a superhighway, but can't flow through the interior of the material at all. It takes a special set of conditions ...

In the latest advance along those lines, researchers at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University systematically probed the "phase transition" in which a TI loses its quantum properties and becomes just another ordinary insulator. They did this by using spiraling beams of laser light to generate harmonics—much like the vibrations of a plucked guitar string—from the material they were examining. Those harmonics make it easy to distinguish what's happening in the superhighway layer from what's happening in the interior and see how one state gives way to the other, they reported in Nature Photonics today.

"The harmonics generated by the material amplify the effects we want to measure, making this a very sensitive way to see what's going on in a TI," said Christian Heide, a postdoctoral researcher with the Stanford PULSE Institute at SLAC, who led the experiments. "And since this light-based approach can be done in a lab with tabletop equipment, it makes exploring these materials easier and more accessible than some previous methods." These results are exciting, added PULSE principal investigator Shambhu Ghimire, because they show the new method has potential for watching TIs flip back and forth between superhighway and insulating states as it happens and in fine detail—much like a using camera with a very fast shutter speed.

The translucent crystal at the center of this illustration is a topological insulator, a quantum material where electrons (white dots) flow freely on its surface but not through its interior. By hitting a TI with powerful pulses of circularly polarized laser light (red spiral), SLAC and Stanford scientists generated harmonics that revealed what happens when the surface switches out of its quantum phase and becomes an ordinary insulator. Credit: Greg Stewart/SLAC National Accelerator Laboratory Read more

Diagram of an experimental setup at SLAC's high-power laser lab where scientists used circularly polarized laser light to probe a topological insulator—a type of quantum material that conducts electric current on its surfaces but not through its interior. A process called high harmonic generation shifts the laser light to higher energies and frequencies, or harmonics, as it passes through a TI. The harmonics allow scientists to clearly distinguish what electrons are doing in the material's conductive surface and its insulating interior. Credit: Shambhu Ghimire/Stanford PULSE Institute Read more

Laser light is usually linearly polarized, meaning that its waves oscillate in only one direction—up and down, in the example at left. But it can also be circularly polarized, at right, so its waves spiral like a corkscrew around the direction the light is traveling. A new study from SLAC and Stanford predicts that this circularly polarized light can be used to explore quantum materials in ways that were not possible before. Credit: Greg Stewart/SLAC National Accelerator Laboratory Read more