Coupled Metamaterial–Phonon Terahertz Range Polaritons in a Topological Insulator

We report terahertz time-domain spectroscopy experiments demonstrating strong light–matter coupling in a terahertz LC metamaterial (MM) in which the phonon resonance of a topological insulator thin film is coupled to the photonic modes of an array of electronic split ring resonators. As we tune the MM resonance frequency through the frequency of the low-frequency α mode of (BixSb1–x)2Te3 (BST), we observe strong mixing and level repulsion between the phonon and MM resonance. This hybrid resonance is a phonon polariton. We observe a normalized coupling strength, η = ΩR/ωc ≈ 0.09, using the measured vacuum Rabi frequency and cavity resonance. Our results demonstrate that one can tune the mechanical properties of these materials by changing their electromagnetic environment and therefore modify their magnetic and topological degrees of freedom via coupling to the lattice in this fashion.