Modification of Terahertz Complex Conductivity of Thin Film BCS Superconductors in High-Q Photonic Crystal Cavities

Novel phenomena in materials are being actively explored in optical cavities in the complete absence of any external fields other than the quantum vacuum, or zero-point, electromagnetic fields. One of the most fascinating proposals is the cavity-induced modifications of the superconducting state. Here, we present the results of our studies on thin superconducting niobium nitride (NbN) films inside high-Q terahertz cavities in search of some of the recently predicted phenomena of cavity-enhanced, cavity-mediated, and cavity-induced superconductivity. We performed terahertz time-domain spectroscopy measurements of the NbN films inside one-dimensional photonic crystal cavities at various temperatures and magnetic fields. We analyzed the optical conductivity of the films in free space based on the Zimmermann formalism and the film transmittance in the cavity using the transfer matrix method with the optical constants obtained in free space. Our results suggest that the terahertz complex conductivity near the optical gap 2Δ of the NbN film is modified when the film is placed inside the cavity. We propose a few possible mechanisms for this phenomenon.