Introducing disorder or granularity into conventional superconductors suppresses the superfluid density and thus constitutes a unique tuning knob for electronic properties. This addresses fundamental questions such as limiting superconductivity by phase coherence of the condensate rather than electron pairing, and it plays an important role for novel applications of superconductors for quantum technologies.

Here we study the electrodynamics of strongly disordered and granular superconducting thin films using GHz and THz spectroscopy. In the THz frequency range we cover frequencies below and above the energy gap of typical disordered superconductors such as transition metal nitrides, and for NbN films grown by atomic layer deposition we have found electrodynamic signatures of a modified density of states that can be modeled by the Dynes formalism.

The GHz range is particularly relevant for electronic applications that operate deep in the superconducting state, and here we investigate granular aluminum thin films. A particular focus is on Corbino reflectometry, which allows us to observe in a single microwave spectrum both the defining signature of superconducting kinetic inductance in the broadband response, namely a linear-in-frequency lossless reactance, as well as a peculiar sequence of resonances that relate to oscillating plasma modes of the superfluid condensate, which are commonly exploited in quantum circuitry.