'Topological plexcitons' could help design solar cells

"When light and matter interact, they exchange energy," explained Joel Yuen-Zhou, an assistant professor of chemistry and biochemistry at UC San Diego and the first author of the paper.

"Energy can flow back and forth between light in a metal (so called plasmon) and light in a molecule (so called exciton). When this exchange is much faster than their respective decay rates, their individual identities are lost, and it is more accurate to think about them as hybrid particles; excitons and plasmons marry to form plexcitons."

Materials scientists have been looking for ways to enhance a process known as exciton energy transfer, or EET, to create better solar cells as well as miniaturised photonic circuits which are dozens of times smaller than their silicon counterparts.

"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 Yuen-Zhou.

The drawback with EET, however, is that this form of energy transfer is extremely short-ranged, on the scale of only 10 nm, and quickly dissipates as the excitons interact with different molecules.

One solution to avoid those shortcomings is to hybridise excitons in a molecular crystal with the collective excitations within metals to produce plexcitons, which travel for 20,000 nm, a length which is on the order of the width of human hair.

Plexcitons are expected to become an integral part of the next generation of nanophotonic circuitry, light-harvesting solar energy architectures and chemical catalysis devices. But the main problem with plexcitons, said Yuen-Zhou, is that their movement along all directions, which makes it hard to properly harness in a material or device.

He and a team of physicists and engineers at MIT and Harvard found a solution to that problem by engineering particles called "topological plexcitons," based on the concepts in which solid state physicists have been able to develop materials called "topological insulators."

"Topological insulators are materials that are perfect electrical insulators in the bulk but at their edges behave as perfect one-dimensional metallic cables," Yuen-Zhou said.

"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. In other words, one may think about the electrons being blind to impurities."

Plexcitons, as opposed to electrons, do not have an electrical charge. Yet, as Yuen-Zhou and his colleagues discovered, they still inherit these robust directional properties. Adding this "topological" feature to plexcitons gives rise to directionality of EET, a feature researchers had not previously conceived.

This should eventually enable engineers to create plexcitonic switches to distribute energy selectively across different components of a new kind of solar cell or light-harvesting device.