Terahertz Cavity Phonon Polaritons in the Deep-Strong Coupling Regime

We have studied the resonant and nonperturbative coupling of transverse optical phonons in lead telluride with photons in small-mode-volume terahertz cavities, observing a giant vacuum Rabi splitting on the order of the uncoupled phonon and cavity frequencies. Our terahertz time-domain spectroscopy experimental data, systematically collected as a function of sample thickness, temperature, and cavity length, can be well reproduced by our electromagnetic simulations. These results demonstrate that this uniquely tunable platform is promising for realizing and understanding predicted cavity-vacuum-induced ferroelectric instabilities, as well as for exploring applications of light-matter coupling in the ultra- and deep-strong coupling regimes in quantum technology.