New types of particles called 'topological plexcitons' have been engineered by researchers in the US, and they could help pave the way for more efficient energy transfers in solar cells and other forms of photonic circuits.

To understand what's different about these new topological plexcitons, we need to get back to basics on some of physics involved with light and matter when they interact on the tiniest of scales.

The University of California, San Diego team has managed to improve on a process known as exciton energy transfer (EET). EET describes the way light and matter exchange energy when they meet.

"Energy can flow back and forth between light in a metal (so called plasmon) and light in a molecule (so called exciton)," said one of the team, biochemist Joel Yuen-Zhou.

"When this exchange is much faster than their respective decay rates," he added, "their individual identities are lost, and it is more accurate to think about them as hybrid particles; excitons and plasmons marry to form plexcitons."

In other words, on its own, EET is only possible over very short distances – about about one hundred millionth of a metre. But one way to extend this is by creating plexcitons, where excitons in a molecular crystal are combined with plasmons – the energy created from light interacting with metal.

That increases the range of EET to about the width of a human hair, but the energy flow is very difficult to harness, which is where this new research comes in.

Physicists from the UC San Diego, the Massachusetts Institute of Technology (MIT), and Harvard University have used materials called topological insulators to act as conductors for EET, forcing the plexcitons to move in one direction, and that means scientists can control the flow of light energy at an incredibly small scale.

"Understanding the fundamental mechanisms of EET enhancement would alter the way we think about designing solar cells or the ways in which energy can be transported in nanoscale materials," said chemist Joel Yuen-Zhou from UC San Diego.

"The exciting feature of topological insulators is that even when the material is imperfect and has impurities, there is a large threshold of operation where electrons that start travelling along one direction cannot bounce back, making electron transport robust," said Yuen-Zhou. "In other words, one may think about the electrons being blind to impurities."

One of the applications of the research is it should enable engineers to create 'plexcitonic switches' that can distribute energy selectively across solar panels or other kinds of light-harvesting devices.

The researchers think plexcitons will be be crucial in the development of light-based nanoelectronics in the future, so being able to control them in this way could be a significant step forward.

Miniaturised photonic circuits have the potential to be dozens of times smaller than today's silicon circuits, so it's possible that topological plexcitons will end up in lot of the devices we use every day. Watch this space.  

The research is published in Nature Communications.