Broadband Near-Field Imaging of Strong Polariton Coupling in Graphene/CrI3 Heterostructures

Interfacial charge transfer in van der Waals heterostructures provides a powerful, contact-free mechanism to activate two-dimensional plasmons. Chromium iodide (CrI3), a layered ferromagnetic insulator, induces significant doping in adjacent graphene, yet its nanoscale polaritonic response and the optical manifestation of this charge transfer remain unexplored. Here, we report the direct real-space observation of propagating graphene surface plasmon polaritons (SPPs) sustained entirely by spontaneous interfacial charge transfer at the graphene/CrI3 interface. Interestingly, a distinct polaritonic anticrossing near 214 cm-1 (6.4 THz) between the charge-transfer-induced graphene SPP and the CrI3 phonon is identified, suggesting this hybrid system lies within the strong-coupling regime. Our work is enabled by the recent advances in broadband synchrotron near-field nanospectroscopy and suggests intriguing opportunities for magnetically switchable polaritonics in the hybrid charge-transfer heterostructures.