Strongly Coupled Magnon–Plasmon Polaritons in Graphene-Two-Dimensional Ferromagnet Heterostructures

Magnons and plasmons are different collective modes, involving the spin and charge degrees of freedom, respectively. Formation of hybrid plasmon–magnon polaritons in heterostructures of plasmonic and magnetic systems faces two challenges, the small interaction of the electromagnetic field of the plasmon with the spins, and the energy mismatch, as in most systems plasmons have energies orders of magnitude larger than those of magnons. We show that graphene plasmons form polaritons with the magnons of two-dimensional ferromagnetic insulators, placed up to to half a micrometer apart, with Rabi splittings in the range of 100 GHz (dramatically larger than cavity magnonics). This is facilitated both by the small energy of graphene plasmons and the cooperative super-radiant nature of the plasmon–magnon coupling afforded by phase matching. We show that the coupling can be modulated both electrically and mechanically, and we propose a ferromagnetic resonance experiment implemented with a two-dimensional ferromagnet driven by graphene plasmons. We acknowledge financial support from the Fundação Para a Ciência e a Tecnologia, Portugal Grant No. PTDC/FIS-MAC/2045/2021) and from the European Union (Grant FUNLAYERS - 101079184). J.F.-R. and A.T.C. acknowledge financial support from the Swiss National Science Foundation Sinergia (Grant Pimag), MICIIN – Spain (Grant No. PID2019-109539GB-41), Generalitat Valenciana (Grant Nos. Prometeo2021/017 and MFA/2022/045), FEDER/Junta de Andalucía-Consejera de Transformación Económica, Industria, Conocimiento y Universidades (Grant No. P18-FR-4834). N.M.R.P. and M.I.V. acknowledge support by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020, COMPETE 2020, PORTUGAL 2020, FEDER, and FCT through projects POCI-01-0145-FEDER-028114, POCI-01-0145-FEDER-02888 and PTDC/NANOPT/29265/2017, PTDC/FIS-MAC/2045/2021, and EXPL/FIS-MAC/0953/2021, and from the European Commission through the project Graphene Driven Revolutions in ICT and Beyond (Ref. No. 881603, CORE 3).