Excitonic negative refraction mediated by magnetic orders

Excitons are prevalent in many optical excitation processes and bosonic quantum phenomena in semiconductors. During their optical transitions, excitons not only emit or absorb photons, but also determine light propagation behaviours within the host medium. While the former has found numerous applications in light-emitting diodes1 and solar cells2, the latter capability of manipulating anomalous light propagation remains largely underexplored. As a quintessential example, negative refraction3-8—one of the most counterintuitive optical phenomena that bends light in the opposite direction to conventional refraction—has yet to be realized in any excitonic medium. Here, we observe excitonic negative refraction in an excitonic ‘indefinite medium’9-13, i.e., a van der Waals magnet namely chromium sulphide bromide (CrSBr)14-19. By harnessing this anomalous capability, we develop an excitonic hyperlens on an integrated nano-photonic chip, whose functionality is intriguingly mediated by the magnetic orders of CrSBr. Particularly, with the versatile tuneability through various degrees of freedom such as magnetic orders demonstrated here, excitons can fully exploit anomalous optical phenomena that are distinct from previous reports in plasmonic or phononic media. This discovery opens up exciting opportunities for sub-diffraction imaging, electromagnetic cloaking, integrated nano-photonics, and magneto-optical devices by leveraging unique properties of excitons in van der Waals magnets.