Phonon-polaritons in the deep-strong coupling regime*

Formation of polaritons in the ultrastrong and deep-strong (DSC) coupling regimes provides opportunities for exploring novel phases of light–matter hybrids as well as for applications in quantum information processing and technology. Phonon-polaritons are particularly interesting as they are expected to be able to modify and control chemical reactions and superconductivity; they are also predicted to induce a new type of ferroelectric phase transitions. Here, we investigate coupling between vacuum photons and phonons in lead telluride in small-mode-volume terahertz cavities. Using metamaterial cavities to enhance vacuum fluctuation fields in the terahertz range, we observed a Rabi splitting whose value exceeded the cavity-phonon frequency, placing us in the DSC regime. We systematically studied the coupling strength as a function of sample thickness, temperature, and cavity length. These experimental results will be discussed in comparison with results of electromagnetic simulations we conducted.