'A more versatile and powerful foundation for future photonic technologies'

The candidate pool for engineered materials that can help enable tomorrow's cutting-edge optical technologies—such as lasers, detectors and imaging devices—is much deeper than previously believed.

That's according to new research from the University of Michigan that examined a class of materials known as topological insulators. These materials have exciting and tunable properties when it comes to how they transit energy and information.

"We see this as a step toward building a more versatile and powerful foundation for future photonic technologies," said Xin Xie, a research fellow in the U-M Department of Physics and lead author of the new study in the journal Physical Review X.

Although these futuristic materials may sound intimidating, their fundamentals are pretty straightforward. Here, the word insulator has the same meaning as it does in elementary electronics: It's a material that blocks the flow of something, for example, wood or rubber in the case of electricity.

A topological insulator is an insulator with a twist. It's mostly an insulator, but it has a special feature where its outer surface, whatever shape it may be, is conductive. This opens up new possibilities for researchers to control the flow of electricity or light and put those to work in new applications.

The U-M team was interested in a particular type of topological insulator that confines conduction to its edge while allowing a single direction of flow. There are comparable materials being developed in electronics and researchers have been interested in expanding that performance to light, said Hui Deng, senior author of the study and U-M professor of physics.

"There's a lot of interest in these systems for transporting photonic information," she said. "They have unidirectional transport and the light can go around defects without scattering."

It'd be natural to assume, given the systems' strict performance criteria, the list of ways to make them would be rather exclusive. In fact, at present, researchers have focused on just a single way to unlock those properties with help from an external magnetic field.

This largely has to do with a physical property of the system called its band gap. This can be thought of as an energetic hurdle inside the system that needs to be cleared for light or electrons to access a conducting state. 

The topological insulator's band gap gives rise to its unique conductive properties. The size of a band gap is also important. Larger band gaps help "protect" the special edge conduction state, Deng said.

Following design principles from their electronic counterparts, researchers working on these photonic topological insulators had largely focused on a single type of a band gap. But the U-M team, which also included Kai Sun, professor of physics, used symmetry analysis and computer simulations to probe other approaches.

In electronics, the band structure is controlled by the crystal structure of the material's atoms. For optical devices, researchers create "photonic crystals" by fabricating arrays of pits, pillars or other periodic nanoscale features that give rise to a system's optical properties, including its band structure and band gap. 

Coupling certain photonic crystal designs to atomically flat 2D materials created the topological insulators the team was after, according to its simulations. But the simulated systems had band structures that were different from that single type previously being focused on. 

That means the design portfolio for this class of topological insulators—called polariton Chern insulators—is unexpectedly broad, the researchers said. For instance, the team took as a case study a type of photonic crystal that's already used in other areas of optical research.

"What surprised me most was how common the required band structures actually are," Xie said. "Even standard photonic crystal designs—long used in other contexts—can readily support polariton Chern insulators, delivering remarkable performance enhancements."

The next step for the team is fabricating real examples of the systems they've simulated, which is no trivial feat, but such experiments are a strength of Deng's team. When such a topological insulator is built in a lab, the researchers' work estimates its band gap could be up to about 100 times larger than the current record. 

The research was funded by the Army Research Office, Air Force Office of Scientific Research, Office of Naval Research, National Science Foundation, and Gordon and Betty Moore Foundation. The study was made possible by the Advanced Research Computing Core at U-M and its Great Lakes high-performance computing cluster.