There is currently much interest in searching for the novel states, phases, and phenomena that are predicted to exist in new regimes of beyond strong light–matter interaction such as ultrastrong and deep-strong coupling regimes. The ultrastrong coupling (USC) regime arises when the light–matter coupling strength, g, becomes a significant fraction of the bare resonance frequency of light and matter, ω0. Furthermore, when g exceeds ω0, the light–matter hybrid enters the deep-strong coupling (DSC) regime. There are many of the new exotic physics expected to occur in the DSC regime that remain unexplored. Here, I will present our recent study on the resonant and nonperturbative coupling of transverse optical phonons in the lead-iodide perovskite (CH3NH3PbI3) and lead telluride (PbTe) with photons in small-mode-volume terahertz metasurface (MS) cavities, observing a giant vacuum Rabi splittings in the Strong, USC and DSC regimes. 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.