Polaritons combine the best of two completely distinct worlds. These hybrid particles combine light and organic molecules, making them suitable energy transfer vessels in organic semiconductors. Because of their photonic origins, they are both compatible with current electronics and move quickly. They are, however, difficult to govern, and much of their behavior remains a mystery.
A team led by Andrew Musser, assistant professor of chemistry and chemical biology in the College of Arts and Sciences, discovered a mechanism to control the speed of this energy flow. This “throttle” can accelerate polaritons to near-light speed, increasing their range – a strategy that could eventually lead to more efficient solar cells, sensors, and LEDs.
“Tuning the Coherent Propagation of Organic Exciton-Polaritons via Dark State Delocalization,” the team’s article, was published in Advanced Science. Raj Pandya of the University of Cambridge is the principal author.
The absolute speed isn’t necessarily important. What is more useful is the distance. So if they can travel hundreds of nanometers, when you miniaturize the device – say, with terminals that are 10’s of nanometers apart – that means that they will go from A to B with zero losses.
Andrew Musser
Musser and colleagues at the University of Sheffield have been investigating a method of producing polaritons using tiny sandwich structures of mirrors called microcavities that trap light and compel it to interact with excitons — mobile bundles of energy composed of a bound electron-hole pair.
They previously shown how microcavities can save organic semiconductors from “dark states” in which they do not generate light, with implications for better organic LEDs. The scientists employed a sequence of laser pulses, which functioned like an ultrafast video camera, to measure how energy traveled through the microcavity structures in real time for the new project. However, the squad encountered its own speedbump. Polaritons are so complicated that even analyzing such observations might be difficult.
“What we discovered was entirely unexpected. We sat on the data for two years, trying to figure out what it all meant,” said Musser, the paper’s senior author. The researchers eventually realized that by adding more mirrors and increasing the reflectivity of the microcavity resonator, they could effectively turbocharge the polaritons.
“The manner we were modifying the speed of motion of these particles is still practically unparalleled in the literature,” he said. “But now, not only have we proved that placing materials into these structures can make states go much quicker and much further, but we now have a lever to really control how fast they go. This gives us a very clear roadmap now for how to try to improve them.”
According to Musser, elementary excitations in common organic materials move at a rate of 10 nanometers per nanosecond, which is about similar to the speed of world-champion sprinter Usain Bolt. That may be fast for humans, but it is a very slow process on the nanoscale, according to him.
The microcavity method, on the other hand, propels polaritons a hundred thousand times quicker – at a velocity on the order of 1% of the speed of light. While the conveyance is brief (less than a picosecond, or roughly 1,000 times shorter), the polaritons travel 50 times further.
“The absolute speed isn’t necessarily important,” Musser said. “What is more useful is the distance. So if they can travel hundreds of nanometers, when you miniaturize the device — say, with terminals that are 10’s of nanometers apart — that means that they will go from A to B with zero losses. And that’s really what it’s about.”
This puts physicists, chemists, and material scientists one step closer to realizing their aim of developing innovative, efficient device architectures and next-generation electronics that are not hampered by overheating.
“Many devices that utilise excitons instead of electrons only operate at cryogenic temperatures,” Musser explained. “However, with organic semiconductors, you may begin to accomplish a wide range of intriguing and fascinating capabilities at room temperature. As a result, these same processes can be used to power new types of lasers, quantum simulators, and even computers. There are numerous applications for these polariton particles if we can better comprehend them.”
