Strong Metasurface–Josephson Plasma Resonance Coupling in Superconducting La_{2− x}Sr_xCuO₄

Terahertz spectroscopy of the c-axis Josephson plasma resonance (JPR) in high-temperature cuprates is a powerful probe of superconductivity, providing a route to couple to and interact with the condensate. Electromagnetic coupling between metasurface arrays of split ring resonators (SRRs) and the JPR of a La_{2− x}Sr_xCuO₄ single crystal (T_c = 32 K) is investigated. The metasurface resonance frequency (ω_MM), determined by the SRR geometry, is swept through the JPR frequency (ω_JPR = 1.53 THz) using a series of interchangeable tapes applied to the same single crystal. Terahertz reflectivity measurements on the resulting hybrid superconducting metamaterials (HSMMs) reveal anticrossing behavior characteristic of strong coupling. The experimental results, validated with numerical simulations, indicate a normalized Rabi frequency of Ω_R = 0.29. Further, it is shown that HSMMs with ω_MM > ω_JPR provide a route to couple to hyperbolic waveguide modes in c-axis cuprate samples. This work informs future possibilities for optimizing the coupling strength of HSMMs and investigating nonlinear superconductivity under high field terahertz excitation.