Electrodynamics of granular aluminum from superconductor to insulator: Observation of collective superconducting modes

We report on a detailed study of the optical response and Tc-ρ phase diagram (Tc being the superconducting critical temperature and ρ the normal state resistivity of the film) of granular aluminum, combining transport measurements and a high resolution optical spectroscopy technique. The Tc-ρ phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion, and the superconducting gap Δ. We provide direct evidence for two different types of well resolved subgap absorptions, at ω₁Δ and at Δω₂2Δ (decreasing with increasing resistivity).