Delocalization unlocks tunability: giant exciton energy shift and ultra-strong light-matter interaction controlled by magnetic field in CrSBr

Optoelectronics strongly benefited of collective bounded electron-hole excitations for applications. In particular, an active field of research aims to look for tuning knobs that associate the optical response of a material to other degrees of freedom. CrSBr is a 2D antiferromagnetic semiconductor, and it attracted a lot of attention due to the unprecedented possibility to tune by 20 meV its first excitonic resonance (in the near infrared) by acting on its magnetic state via modification of the interlayer electronic coupling.

In this talk, we discuss the presence of a second exciton (in the visible range), taking advantage of several optical spectroscopy probes. We discover a 100 meV energy shift driven by an external magnetic field, due to a very delocalized excitation that is strongly influenced by the spin splitting of the band structure. Based on experimental data, we propose two different physical origins for the two resonances, highlighting how CrSBr excitons are placed at the crossover between onsite (Frenkel) and delocalized (Wannier) character. Taking advantage of its very large and anisotropic refractive index, we also exploit CrSBr as a cavity and to observe the formation of exciton-polaritons with a Rabi splitting of about 370 meV. These results clarify the nature of excitons in CrSBr and open perspectives to optimize magneto-electronic coupling for nanophotonics applications.