Terahertz Magnon Polaritons in a Cavity

Hybridized light-matter states, or polaritons, have recently attracted attention for their potential as a platform for quantum information processing applications and investigations into new exotic physics. In particular, strong coupling between collective spin excitations and cavity photons offers excellent potential for cavity-enabled devices and spintronics. Antiferromagnetic magnons are a promising channel due to high characteristic frequencies and insensitivity to magnetic perturbations. However, obtaining strong coupling in antiferromagnetic resonances has proven to be experimentally challenging due to the higher frequency scale, difficulty of detuning, and typical low coupling strength ratio to the matter decay rate. Using terahertz time-domain spectroscopy in magnetic fields up to 25 T, we observe cavity magnon-polaritons at frequencies greater than one terahertz in an antiferromagnetic slab of NiO forming a Fabry–Pérot cavity.