In the mysterious microscopic realm where the electromagnetic fields of light and matter intimately intermingle as they exchange energy, plasmons, excitons, and other particles with unexpected and usual properties abound. Now physicists have created a new set of energy-carrying particles to add to this range. Dubbed "topological plexcitons," these new particles show promise in greatly enhancing energy flows for solar cells and nanoscale photonic circuitry.

Scientists at UC San Diego, MIT and Harvard University have engineered these particles to help improve a process known as exciton energy transfer (EET). Created as a conjunction of plasmons (a quantity of collective electron oscillations) and excitons (excited electrons bound to the hole produced by their excitation), topological plexcitons specifically aid better direct energy flows in the EET.

"When light and matter interact, they exchange energy," said Joel Yuen-Zhou, assistant professor of chemistry and biochemistry at UC San Diego. "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."

Plexcitons travel for 20,000 nanometers, a length which is on the order of the width of human hair(Credit: Joel Yuen-Zhou)

EET has previously only been possible over picayune distances, in the order of about 10 nanometers (a 100 millionth of a meter), and rapidly disappeared as the excitons' energy fields intermingled – then dissipated – as they encountered other particles and molecules along the way.

One method physicists used to solve this problem was to combine excitons in a molecular crystal with the collective excitations within metals to create plexcitons. This improved the problems of rapid dissipation, and increased the distance for EET to around 20,000 nanometers (about the width of a human hair). Whilst this may seem a tad small, it was in fact a vast improvement at such minuscule scales.

Unfortunately, whilst plexcitons helped increase the distance of energy transfer, the direction in which they did so was uncontrolled, which tended to negate any real gains. The solution to this, the researchers found, was to create the new topological plexcitons designed around the properties of materials known as "topological insulators" that act as insulators internally, but have surfaces that conduct, so that electrons are only able to move along the surface of the material.

"Topological insulators are materials that are perfect electrical insulators in the bulk but at their edges behave as perfect one-dimensional metallic cables," said Yuen-Zhou. "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 traveling along one direction cannot bounce back, making electron transport robust. In other words, one may think about the electrons being blind to impurities."

The directionality created on plexcitons by adding a toplogical component to the mix means that one day "plexcitonic" switches could be created to selectively distribute energy across the microscopic components of new types of solar cells or other light-harvesting devices to greatly improve power flows and efficiencies.

The results of this research were recently published in the journal Nature Communications.

Source: UC San Diego

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