Measurement of optical absorption in granular aluminum thin films at room temperature

Because of their versatility and scalability, Microwave Kinetic Inductance Detectors (MKIDs) represent a rapidly developing class of cryogenic superconducting detectors. In the context of optical photon detectors, the efficiency of MKIDs faces limitations due to the reflection of visible and near-infrared (VIS-NIR) photons by low resistivity metallic films, which prevents a significant portion of incident photons from being absorbed and detected. To address this issue, we propose the use of granular Aluminum (grAl), a disordered superconductor which can achieve resistivity as high as 10 m$\Omega$$^.$cm. We measure optical transmission and reflection of grAl thin films in the wavelength range 400 nm to 1100 nm, from which we infer their absorption. Compared to other commonly used superconductors in the MKIDs community, for grAl we observe comparable or superior absorption, depending on the wavelength and the film thickness, which makes it a promising alternative material.