Anomalous response of superconducting titanium nitride resonators to terahertz radiation

Abstract

We present an experimental study of KIDs fabricated of atomic layer deposited TiN films, and characterized at radiation frequencies of 350 GHz. The responsivity to radiation is measured and found to increase with increasing radiation powers, opposite to what is expected from theory and observed for hybrid niobium titanium nitride / aluminium (NbTiN/Al) and all-aluminium (allAl) KIDs. The noise is found to be independent of the level of the radiation power. The noise equivalent power (NEP) improves with higher radiation powers, also opposite to what is observed and well understood for hybrid NbTiN/Al and all-Al KIDs. We suggest that an inhomogeneous state of these disordered superconductors should be used to explain these observations. Superconducting resonators have been proposed as kinetic inductance detectors (KIDs) for sensitive multipixel radiation detection [1]. Antenna-coupled hybrid niobium titanium nitride / aluminium (NbTiN/Al) KIDs [2,3] and all-aluminium (all-Al) [4] have shown generationrecombination noise and photon noise limited performance. KIDs can also be constructed as lumped element kinetic inductance detectors (LEKIDs) [5] in which the KID is arranged as a photon absorbing area matched to free space. Aluminium LEKIDs have also shown generation-recombination and photon noise limited performance [6]. However, the low normal-state resistivity of Al makes the design of the absorber for very high frequency radiation complex. Therefore, superconductors with a high normal state resistivity have recently become of particular interest [9].A figure of merit F to optimize the responsivity of KIDs is defined as, with α sc the kinetic inductance fraction, τ the quasiparticle recombination time, Q i the internal quality factor, F res the resonance frequency, N (0) is the single spin electron density of states at the Fermi level, and V the volume of the KID. For example, Al KIDs have a long quasiparticle recombination time (a few milliseconds) and high internal quality factors (above one million), but their kinetic inductance fraction is low and their volume large.Superconducting materials with a high resistivity in the normal-state are promising because of their high quality factor and a long enough relaxation time. The high normal resistance implies a large sheet inductance, resulting in a large kinetic inductance fraction, which lowers the KID volume. The high surface impedance also eases matching to free space and optimises the photon absorption. Given the high quality NbTiN resonators pioneered by Barends et al. [7,8], titanium nitride (TiN) has been proposed, because it has the previously mentioned properties in addition to a tuneable critical temperature, which facilitates a relatively long quasiparticle lifetime [9]. Currently, several groups are studying the implementation of TiN KID devices and instruments [10][11][12][13][14]. However, a material like TiN has also drawn the attention of the condensed matter physics community, interested in the disorder-induced superconductorto-insulator transitio...

Design and Characterisation of Titanium Nitride Subarrays of Kinetic Inductance Detectors for Passive Terahertz Imaging

“…Similar behaviour of amplitude responsivity was observed by authors of Ref. [11]. This is opposite to the behaviour of responsivity observed in Al MKIDs [20] where reduction of quasiparticle lifetime at higher quasiparticle densities causes responsivity to decrease with increasing .…”

“…As noted by authors of Ref. [11], such anomalous behaviour is possibly due to spatial non-uniformity of superconducting gap energy in highly disordered TiN films. According to this conjecture, at lower optical powers, excess quasiparticles are trapped in low-gap areas, but with increasing optical powers, their freedom to move increases.…”

The effect of magnetic field on the intrinsic detection efficiency of superconducting single-photon detectors

“…It would have to take into account the direct effect of the magnetic field on the vortex barrier, the current distribution in our sample in the presence of vortices, and the associated local changes in D. Any full microscopic theory of photodetection in SSPDs, even at zero magnetic field, would also need to take into account the intrinsic inhomogeneity that has been observed in similar films, 35,36 and the observed intrinsic pair breaker, as it has been shown recently that these can give rise to an unexpected response to electromagnetic radiation. 37 We have demonstrated that for low fields, the response of an SSPD to an applied magnetic field is set entirely by the effect that the field has on the electronic state of the material. In this regime, there is an interchange between bias current and applied magnetic field, in agreement with the homogeneous theory for dirty-limit superconductivity.…”

Bi-layer kinetic inductance detectors for space observations between 80–120 GHz

“…Therefore we need to adopt lower superconducting gap films. Several authors have investigated titanium nitride (TiN) as an alternative to Al KID (Swenson et al 2013;Leduc et al 2010;Bueno et al 2014) in the past years. In parallel, we have studied a large number of possible alternative solutions.…”