NREL Team Observes Hyperbolic Exciton Polaritons in Novel Material.

A collaborative team comprising researchers from the National Renewable Energy Laboratory (NREL), Columbia University, King’s College London, the University of Washington, the Flatiron Institute, and Radboud University, has observed hyperbolic exciton polaritons (HEPs) in the van der Waals magnet chromium sulfide bromide (CrSBr), as detailed in a recent publication in Nature Communications. The investigation, utilising scanning near-field optical microscopy, revealed that CrSBr, a layered material, exhibits hyperbolic behaviour wherein the dielectric functions display opposite signs along differing axes – a characteristic of hyperbolic metamaterials. This anisotropy facilitates unique control over the propagation of both light and excitons – bound electron-hole pairs – resulting in the formation of HEPs, exotic quasiparticles arising from strong coupling between light and matter. NREL’s Mark van Schilfgaarde, a chief theorist and co-author, highlights the significance of this observation as a step towards harnessing these properties for potential technological applications in optoelectronic devices, leveraging the ability to manipulate exciton movement for innovations in electricity generation and fuel creation.

Exciton Polariton Discovery

Researchers at the National Renewable Energy Laboratory (NREL), Columbia University, King’s College London, the University of Washington, the Flatiron Institute, and Radboud University have reported the observation of hyperbolic exciton polaritons in the van der Waals magnet, chromium sulfide bromide (CrSBr). This finding, published in Nature Communications, represents a significant departure from conventional light-matter interactions and potentially enables novel optoelectronic functionalities. Exciton polaritons are quasiparticles arising from the strong coupling of light and excitons – bound electron-hole pairs.

The study employed scanning near-field optical microscopy to characterise the behaviour of these quasiparticles within the layered CrSBr material, a member of the van der Waals family known for its two-dimensional structure and unique electronic properties. The observed hyperbolic exciton polaritons (HEPs) are particularly noteworthy due to their anisotropic dielectric functions, exhibiting opposite signs along different crystallographic axes – a characteristic trait of hyperbolic metamaterials.

This anisotropy facilitates exceptional control over the propagation of both light and excitons, offering a pathway to manipulate their movement and interactions with unprecedented precision. NREL’s Mark van Schilfgaarde, a chief theorist and co-author of the study, highlights that HEPs possess attractive properties for technological applications, though the precise nature of these applications remains an area of ongoing investigation.

Conventional light-matter interactions are typically weak, resulting in only minor perturbations to a material’s quantum state. However, strong coupling – achieved in this case within CrSBr – allows for the formation of polaritons, where excitons influence their velocity and spatial confinement.

NREL’s Mark van Schilfgaarde, a chief theorist and co-author of the study, highlighted the attractive properties of these HEPs, suggesting their potential for technological exploitation. The ability to manipulate these quasiparticles through material design and external stimuli offers a pathway towards developing novel optoelectronic devices with enhanced performance characteristics.