Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems. 

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Andrey Baydin, Kenji Hayashida, Takuma Makihara, Fuyang Tay, Xiaoxuan Ma, Wei Ren, Guohong Ma, G. Timothy Noe II, Ikufumi Katayama, Jun Takeda, Hiroyuki Nojiri, Shixun Cao, Motoaki Bamba, Junichiro Kono

Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons

arXiv:2208.10030