Polaritons offer the best of two very different worlds. These hybrid
particles combine light and molecules of organic material, making them ideal
vessels for energy transfer in organic semiconductors. They are compatible
with modern electronics but also move speedily, thanks to their photonic
origins.
However, they are difficult to control, and much of their behavior is a
mystery.
A project led by Andrew Musser, assistant professor of chemistry and
chemical biology in the College of Arts and Sciences, has found a way to
tune the speed of this energy flow. This "throttle" can move polaritons from
a near standstill to something approaching the speed of light and increase
their range—an approach that could eventually lead to more efficient solar
cells, sensors and LEDs.
The team's paper, "Tuning the Coherent Propagation of Organic
Exciton-Polaritons through Dark State Delocalization," published April 27 in
Advanced Science. The lead author is Raj Pandya of the University of
Cambridge.
Over the last several years, Musser and colleagues at the University of
Sheffield have explored a method of creating polaritons via tiny sandwich
structures of mirrors, called microcavities, that trap light and force it to
interact with excitons—mobile bundles of energy that consist of a bound
electron-hole pair.
They previously showed how microcavities can rescue organic semiconductors
from "dark states" in which they don't emit light, with implications for
improved organic LEDs.
For the new project, the team used a series of laser pulses, which
functioned like an ultrafast video camera, to measure in real time how the
energy moved within the microcavity structures. But the team hit a speedbump
of their own. Polaritons are so complex that even interpreting such
measurements can be an arduous process.
"What we found was completely unexpected. We sat on the data for a good two
years thinking about what it all meant," said Musser, the paper's senior
author.
Eventually the researchers realized that by incorporating more mirrors and
increasing the reflectivity in the microcavity resonator, they were able to,
in effect, turbocharge the polaritons.
"The way that we were changing the speed of the motion of these particles is
still basically unprecedented in the literature," he said. "But now, not
only have we confirmed that putting materials into these structures can make
states move much faster and much further, but we have a lever to actually
control how fast they go. This gives us a very clear roadmap now for how to
try to improve them."
In typical organic materials, elementary excitations move on the order of 10
nanometers per nanosecond, which is roughly equivalent to the speed of
world-champion sprinter Usain Bolt, according to Musser.
That may be fast for humans, he noted, but it is actually quite a slow
process on the nanoscale.
The microcavity approach, by contrast, launches polaritons a
hundred-thousand times faster—a velocity on the order of 1% of the speed of
light. While the transport is short lived—instead of taking less than a
nanosecond, it's less than picosecond, or about 1,000 times briefer—the
polaritons move 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 brings physicists, chemists and material scientists ever closer to
their goal of creating new, efficient device structures and next-generation
electronics that aren't stymied by overheating.
"A lot of technologies that use excitons rather than electrons only operate
at cryogenic temperatures," Musser said. "But with organic semiconductors,
you can start to achieve a lot of interesting, exciting functionality at
room temperature. So these same phenomena can feed into new kinds of lasers,
quantum simulators, or computers, even. There are a lot of applications for
these polariton particles if we can understand them better."
Reference:
Raj Pandya et al, Tuning the Coherent Propagation of Organic
Exciton‐Polaritons through Dark State Delocalization, Advanced Science
(2022).
DOI: 10.1002/advs.202105569
Tags:
Physics