Tunable phonon-magnon polaritons at around 1.0 THz

The recent growth of interest in light-matter coupling in magnetic materials is motivated by applications in quantum technologies and interesting physics like the observation of dissipative coupling. However, this research is mostly done with ferromagnets in the microwave range. The antiferromagnetic resonance in the terahertz (THz) range is much less explored. Magnons in the THz range can be coupled with other excitations of solid-state, like phonons. Previously, we showed the coupling of magnons in two distant crystals mediated by electromagnetic cavity modes at about 0.25 THz [1]. Here, we show cavity-mediated magnon-phonon coupling at about 1.0 THz. Two parallel-plane slabs of different materials with hundreds of μm thicknesses, hosting exclusively either phonon or magnon around 1.0 THz, were placed next to each other at a tunable gap of up to a few mm, forming a Fabry-Pérot-type cavity. We chose NiO as the material hosting a narrow magnon mode, tunable using temperature around room temperature in the range of 0.7-1.0 THz, and ceramics of Cu₂B₂O₆/CuB₂O₄ (CBO) as the material with a phonon mode at 0.92 THz. When the magnon has a frequency close to that of the phonon in CBO, and the gap between the slabs is so chosen that one of the cavity modes has a close frequency, the resonances in both materials hybridize. Coupling magnons and phonons enhances magnon-photon coupling strength because of the phonon's much higher dipolar moment than that of the magnon. Linewidths of observed phonon-magnon polaritons are narrower than pure phonon-polaritons thanks to the hybridization with magnons. Such hybrid excitations offer a possibility for tuning phonon-polaritons using antiferromagnetic magnons or enhancing magnon-photon coupling strength using phonons.

[1] M. Białek et al, Phys. Rev. Applied 19, 064007 (2023)