  Hybrid light-matter coupled states, or polaritons, in magnetic materials have
attracted significant attention due to their potential for enabling novel
applications in spintronics and quantum information processing. However, most
studies to date have been carried out for ferromagnetic materials with magnon
excitations at gigahertz frequencies. Here, we have investigated strong
resonant photon-magnon coupling at frequencies above 1 terahertz for the first
time in a prototypical room-temperature antiferromagnetic insulator, NiO,
inside a Fabry-P\'erot cavity. The cavity was formed by the crystal itself when
it was thinned down to an optimized thickness. By using terahertz time-domain
spectroscopy in high magnetic fields up to 25 T, we swept the magnon frequency
through Fabry-P\'erot cavity modes and observed photon-magnon anticrossing
behavior, demonstrating clear vacuum Rabi splittings exceeding the polariton
linewidths. These results show that NiO is a promising platform for exploring
antiferromagnetic spintronics and cavity magnonics in the terahertz frequency
range.
