Measurements of strong coupling between metamaterials and superconducting Josephson plasmons in La_2-xSr_xCuO₄ at terahertz frequencies

We experimentally investigate the coupling between an array of metamaterial (MM) resonators and the Josephson Plasma Resonance (JPR) in superconducting La_2-xSr_xCuO₄ using terahertz (THz) time domain spectroscopy. The MM can be placed within ~1μm of the LSCO surface (the c-axis lies in the sample plane) by fabricating the MM/polyimide films that can be adhered and detached from the surface, allowing for the possibility of measuring different MM resonators and variations in the MM-LSCO-surface spacing. The electrodynamic response is characterized by a plasma-edge-like feature that is highly renormalized compared to the bare crystal JPR. Measurements as a function of temperature and THz field strength demonstrate how the renormalized response reports strong coupling between the MM resonance and the JPR. Theoretical models provide insight into the observed response, indicating that the edge-like feature is due to splitting of the JPR response from longitudinal coupling with the resonators. Our analysis further elucidates how the upper polariton is not seen in the measured reflectivity, consistent with previously published results [1].

[1] J. S. Schalch et al., Adv. Opt. Mater. 7, 1900712 (2019)