Orateur
Description
Kinetic inductance detectors (KIDs) are state-of-the-art detectors used for observations in the millimeter-wave range in astrophysics. They are planar resonant circuits consisting of thin superconducting films deposited on an insulating substrate. The photon detection principe consists in monitoring the shift in the resonance frequency, which is proportional to the incident power [1].
Instead of using KIDs made from known materials to study unknown astrophysical sources [2], we use known sources to study the spectroscopic responses of KIDs made from various superconductors. The advantages of this technique over conventional optical spectroscopic techniques are: high sensitivity to variations in superfluid density down to a few tens of broken Cooper pairs, a measurement temperature that can go as low as 100 mK, and statistical control of the response through the simultaneous measurement of a large number of KIDs. The optical dilution refrigerator enables illumination at room temperature using various sources, ranging from a broadband Fourier transform spectrometer to monochromatic sources in the Hz to THz range.
First, I will explain how kinetic inductance detectors work and describe the different types of measurements we can perform in the laboratory. I will then present results showing sub-gap modes in granular aluminum [3] and amorphous indium oxide [4]. These sub-gap modes are non-equilibrium phenomena triggered by incident radiation. I will discuss how these modes could explain the additional dissipation observed in disordered thin superconductors [5].
[1] P. K. Day et al, Nature, vol. 425, 817 (2003).
[2] A. Monfardini and G. Lagache, Nature Astronomy 5, 970 (2021).
[3] F. Levy-Bertrand et al, Phys. Rev. B 99, 094506 (2019).
[4] O. Dupré et al, Supercond. Sci. Technol. 30, 04007 (2017).
[5] T. Charpentier et al, arXiv:2507.08953 (2025).