We demonstrate the single photon counting mode at 405 and 850 nm with stoichiometric TiN-based microwave kinetic inductance detectors realized on a sapphire substrate and operated at bath temperatures over 300 mK. The detectors use single 15–25 nm-thick TiN layers featuring a critical temperature in the 2–3 K range. We found that the energy-resolving power R=E/ΔE exhibits an optimum with bath temperature, occurring in the 300–450 mK range, which can be almost double compared to those obtained at the lowest temperatures. Furthermore, the single photon regime is observed up to 700 mK. In addition to a high-temperature operation, the single stoichiometric layer would allow achieving a better uniformity in the critical temperature and, thus, kinetic inductance, compared to the often desired ∼1 K sub-stoichiometric TiN.

Following optical pulses (λ=405 nm) on titanium nitride microwave kinetic inductance detectors cooled down at temperatures T≤Tc/20 (Tc≃4.6 K), we observe a large phase-response highlighting two different modes simultaneously that are nevertheless related. The first corresponds to the well-known transition of cooper-pair breaking into quasi-particles, which produces a known phase response. This is immediately followed by a large inverse response lasting several hundreds of microseconds to several milliseconds depending on the temperature. We propose to model this inverse pulse as the thermal perturbation of the superconductor and interaction with a two-level system (TLS) that reduces the dielectric constant, which in turn modifies the capacitance and, therefore, the resonance frequency. The ratio of the TLS responding to the illumination is on the order of that of the area of the inductor to the whole resonator.

To improve the optical coupling in Microwave Kinetic Inductance Detectors (MKIDs), we investigate the use of a reflective plate beneath the meandered absorber. We designed, fabricated and characterized high-Q factors TiN-based MKIDs on sapphire operating at optical wavelengths with a Au/Nb reflective thin bilayer below the meander. The reflector is set at a quarter-wave distance from the meander using a transparent Al 2 O 3 dielectric layer to reach the peak photon absorption. We expect the plate to recover undetected photons by reflecting them back onto the absorber.

Dans cet article, nous présentons les MKIDs, pour « Microwave Kinetic Inductance Detectors », une nouvelle technologie de détecteur supraconducteur ultrasensible capable de détecter le photon et permettant de couvrir un large spectre électromagnétique allant des ondes millimétriques-submillimétriques aux rayons gamma, en passant par l’optique.