Magnetic Order Unlocks Optical Access to Dark Excitons in CrSBr

TL;DR

This paper demonstrates that magnetic order in CrSBr enables optical access to dark excitons via exciton-magnon coupling, revealing a new method to manipulate quantum states in magnetic semiconductors.

Contribution

It introduces a mechanism where magnetic order makes dark excitons optically accessible through hybridization with magnons in a van der Waals semiconductor.

Findings

Dark exciton at 1.46 eV becomes visible via exciton-magnon coupling.

Active control of hybrid dispersion through high-energy excitation.

Strong renormalization and enhancement of exciton-magnon interactions.

Abstract

Hybrid quasiparticles that intertwine magnetic and electronic degrees of freedom underpin emerging strategies for manipulating and transducing quantum information in solids. A key missing element has been the ability to optically access dark excitons - optically forbidden but functionally crucial states that shape energy flow, coherence, and spin dynamics in quantum materials. Here we show that exciton-magnon coupling provides an optical gateway to dark excitons in the antiferromagnetic van der Waals semiconductor CrSBr. Broadband femtosecond reflectivity reveals a dark exciton at 1.46 eV that is entirely absent in static optical spectra but becomes visible through its coherent hybridization with a GHz magnon. High-photon-energy excitation further allows active control of the hybrid dispersion, enabling strong renormalization and selective enhancement of exciton-magnon interactions. These results establish a general mechanism by which magnetic order renders dark excitons optically addressable, opening a pathway toward engineered hybrid spin-exciton platforms for microwave-to-optical quantum transduction.