With their negative mass, hybrid exciton–polariton particles surprise.

These strange light-matter hybrid particles have gained yet another strange dimension thanks to the unexpected discovery of exciton–polaritons with negative mass.

Dr. Matthias Wurdack, Dr. Tinghe Yun and Dr. Eliezer Estrecho from the Branch of Quantum Sciences and Innovation (QST) were trying different things with exciton polaritons when they understood that under specific circumstances the scattering became upset — comparing to a negative mass.

To add to the astonishment, the surprising reason has ended up being misfortunes.

Dr. Wurdack, who is also a QST member, stated, “We did not expect that—the dispersion followed very unconventional behavior.”

In a semiconducting crystal, a pairing of an electron and a hole results in the formation of an exciton. An exciton–polariton, a hybrid particle, can emerge if this pair then strongly couples to a photon.

Researchers from the ANU and FLEET: from left Dr Eliezer Estrecho, Dr Matthias Wurdack, Dr Tinghe Yun. Credit: Phil Dooley ANU

Exciton–polaritons have been shown to have remarkable properties, such as superfluidity and the formation of Bose-Einstein condensates, since they were first observed more than 30 years ago. In the hope that these bizarre particles will serve as the foundation for future low-energy technologies, a number of research groups around the world are examining their counterintuitive behavior.

The scientists led by Professor Elena Ostrovskaya went back to the whiteboard after making the negative mass observation to try to figure out what might cause such an effect.

Because the experiment consisted of a single tungsten disulfide monolayer embedded in a microcavity, it could be approximated using a straightforward coupled-oscillator model, eliminating any potential for unforeseen effects. The team quickly realized that their calculations matched their observations with an imaginary coupling constant known as dissipative coupling.

However, until they looked at other fields like microwave cavities and realized that an imaginary coupling constant could be equivalent to losses from the light-matter system, they were unable to understand the mathematical model.

The team contacted specialists in numerical modeling of physics in two-dimensional systems, led by Professor Andreas Knorr from the Technical University of Berlin in Germany, to verify that interpretation. Manuel Katzer and Dr. Malte Selig of his group were able to confirm that losses, specifically interactions between exciton–polaritons and phonons in the atomically thin semiconductor, could account for the negative mass.

In addition, the calculations demonstrated that the mass changed from positive to negative when the exciton–phonon interactions were disabled, a pattern that was replicated in subsequent ANU experiments. The outcomes are presently distributed in Nature Correspondences.

Dr. Wurdack compared exciton–polaritons’ behavior to that of boats on a lake.

“Like waves on the lake, the phonons in the active material can alter the boats’ movement, redirecting them or causing them to sway or capsize. “He stated that this decreases their kinetic energies, which ultimately results in losses in momentum, energy, and occupation numbers.”

Dr. Estrecho, who is also from FLEET, stated that scientists go to great lengths to minimize or eliminate losses in the pursuit of low-energy technology.

He stated, “In most cases, we don’t want losses, but here, losses have given us something new.” We were able to apply a negative mass to the strongly coupled system as a result.”

Dr. Wurdack stated that the discovery is yet another aspect of the counterintuitive behavior of exciton–polaritons.

“This behavior could be used for dispersion engineering, possibly to investigate novel physics and create devices that have never been imagined before. “I am hopeful that this will be explored in future research activities, but I cannot yet predict the extent of future applications using this type of dispersion engineering,” he stated.

More information: M. Wurdack et al, Negative-mass exciton polaritons induced by dissipative light-matter coupling in an atomically thin semiconductor, Nature Communications (2023). DOI: 10.1038/s41467-023-36618-6

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