Written by Renée Haran
Plexcitons arise from the close relationship of two quasiparticles: excitons (an excited electron and its resultant positively charged hole acting as a single unit) and plasmons (waves of electrons on a metal’s surface when illuminated), resulting in a new hybrid light-matter state.
While plexcitons have been understood and studied at an increasing rate for the last twenty years, their nanoscale emission properties have remained elusive until now.
Using Foundry-developed scanning near-field optical microscopy (SNOM), a team of researchers led by postdoctoral scientist Junze Zhou and guided by Alexander Weber-Bargioni have successfully achieved high-resolution nanoscale imaging of plexcitons under open-air conditions. Potential applications of the team’s findings include energy-efficient lasers, secure quantum communications systems, ultrafast quantum computers, solar cells, and biochemical assays.
Their success was made possible through collaboration with P. André D. Gonçalves, formerly from the Nanophotonics Theory Group at ICFO – The Institute of Photonic Sciences in Spain (led by co-author F. J. García de Abajo). The researchers recently published their findings in Nature Communications.
“We’ve seen the optical response of this kind of nanostructure for the first time at nanometer scale,” Gonçalves said.
Previous systems often embedded 2D materials within or beneath nano trenches, making them inaccessible to near-field probes. Those setups left the native length properties of plexciton emission largely unexplored. To overcome this challenge, researchers at the Foundry worked with the Nanophotonic Theory Group in Spain to design an innovative platform that positioned the 2D tungsten material directly atop golden nano trenches.
Nano trenches are like the parallel grooves on a vinyl record, but instead of plastic, they’re etched into a gold surface. Rather than using a record player’s needle, scientists read these trenches with a pyramid-shaped probe that uses a 633-nanometer laser beam transmitted through glass fiber. To resolve the nano-properties of plexcitons, the near-field probe is placed nanometers away from the sample surface without physically touching it; the 2D tungsten material sample sitting atop the golden trench is excited with the laser. Once the sample is excited with light, the near-field probe can record the photoluminescence emission given off by the plexcitons.
“This research began with unexpected outcomes,” said Zhou, who originally set out to study excitons, a component of plexcitons, at high resolution to capture their nanoscale properties. Zhou knew the gold nano trenches could boost light emission by acting like tiny antennae, but he hadn’t expected to observe plexcitons in the process.
The researchers realized that if the resonance energies of the plexciton’s two fundamental components – excitons and plasmons – were matched, they could achieve robust visualization of plexcitons, too. “So, the plexciton is a bit like a marriage between the exciton and the plasmon,” said Gonçalves. “And there are some unique advantages to ‘being married’ in this case that you wouldn’t get otherwise.”
Unique plexciton properties include stability at room temperature and shifting into various hybrid configurations that can be more exciton than plasmon and vice versa, enabling the control and manipulation of light at nanoscale.
The collaboration between particles seemed to inspire the researchers themselves: the theoretical and experimental teams worked together to fine-tune the plasmonic resonance by adjusting the nano trench width by just a few nanometers.
“This work shows how the theory and experiment working together is sometimes challenging, but very rewarding,” recalled Gonçalves. “You encounter unexpected results, but when you dig deeper, you discover that things you hadn’t considered actually matter a great deal.”
Interested in Becoming a Foundry User?
Join our collaborative, multidisciplinary environment.
Learn more >
Once the researchers matched the resonance of the nano trenches, they observed the hallmark of energy splitting in the far-field absorption measurement, a clear signature of strong coupling between excitons and plasmons.
When the researchers refined their theoretical calculations to match the actual V-shaped trenches with slanted walls (rather than assuming rectangular trenches), the theory aligned better with experimental data. This gave them deeper insight into how the trenches’ geometry affects their novel system’s properties.
An additional breakthrough came in the form of nano-photoluminescence mapping. Using the SNOM system, the partners created an ultra-detailed map of plexciton emissions, capturing their spatial properties and unique polarization characteristics directly at their intrinsic length scale.
“This groundbreaking mapping represents a significant leap forward,” said Zhou. By unlocking the intriguing ways light interacts with materials through unique hybrid states like plexcitons, researchers are setting the stage for advancements in technologies that use light at tiny scales.
