Ultrastrong Coupling Between Electron Paramagnetic Resonance and Fabry–Pérot Cavity Modes

Ultrastrong light–matter coupling (USC) is a burgeoning field of research due to its possibility of creating new phases of matter, as well as its many potential applications in quantum information technology. Over the last decade, a myriad of work has demonstrated the USC regime in a number of semiconductors, magnets, and superconducting circuits, including recent demonstrations of the deep-strong coupling regime. In this work, we examine the efficacy of using the geometry of a crystal itself as a Fabry–Pérot (FP) cavity. We utilize Gd3Ga5O12 (GGG), a frustrated magnetic insulator possessing a strong electron paramagnetic resonance (EPR) in the presence of an external magnetic field. Compared to other magnetic resonances, such as ferromagnetic resonance and magneto-plasmon resonance, which can be treated as bosonic excitations, EPR retains a fermionic character and its coupling strength depends on the magnetic field and temperature. We optimized the thickness of GGG samples to utilize FP resonances as cavity modes and performed terahertz time-domain spectroscopy experiments in magnetic fields up to 30 T. Our results showed vacuum Rabi splitting of the order of a cavity mode frequency, indicating that the system entered the USC regime. Transfer matrix calculations agree well with experimental data.