Carnegie Mellon University

Two-dimensional tunable bilayers

March 02, 2020

New Research Shows Promise of Tunable Two-Dimensional Materials for Light Detection

By Ben Panko

New research from Carnegie Mellon University shows how two-dimensional materials can be precisely tuned to act as sensitive detectors for a difficult-to-measure form of light.

"Material design sounds like a very complicated topic," said Professor of Physics Di Xiao. "But deep down it's just about how you arrange atoms."

In a new study published in the journal Physical Review Letters, Xiao and Carnegie Mellon postdoctoral fellow Yang Gao show how arranging two layers of graphene atoms can allow the detection of circularly polarized light.

Compared to the more familiar linearly polarized light, in which the waves of radiation move along a plane in a single direction, circularly polarized light occurs when the waves constantly rotate while moving along a plane in space. When polarized this way, light waves rotate left or right, leading to "left-handed light" and "right-handed light."

"To be able to extract this polarization would be very important for many optoelectronic applications," Xiao said, noting that it's currently difficult and expensive to detect circularly polarized light, particularly in the infrared range.

"The relative position between atoms can give you very different properties," Xiao explained, citing how both durable diamonds and soft graphite are composed entirely of carbon atoms but just arranged in different structures.

In the study, Gao took two layers of graphene atoms and changed their arrangement by "twisting" them — essentially rotating the layers in relation to each other. The resulting structure is chiral, meaning it is different from its mirror image. In their study, Gao and Xiao showed that circularly polarized light can induce an electric dipole across the two layers of graphene. Opposite circular polarization of light leads to dipoles in the opposite direction. As the angle of twist changed, the resonance peak of the signal changed in a consistent manner from visible to infrared light, thus creating a material that's "tunable" to certain frequencies of light.

"What I have done is for relatively big angles, so the very next step is to go to smaller angles," Gao said of his future research plans. This should allow even smaller frequencies of light to be detected.

The other author on the study was Carnegie Mellon postdoctoral fellow Yinhan Zhang. The work was funded by a grant from the U.S. Department of Energy and a Simons Foundation Fellowship.