Photon-counting with single stoichiometric TiN layer-based optical MKIDs

Abstract

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.

Investigation of quasi-particle relaxation in strongly disordered superconductor resonators

“…We have also obtained an analytical solution of RT equations for superconductors with relatively slow relaxation at low temperatures with various phonon-trapping factors. We further considered QP diffusion in the superconductor and fitted the diffusion coefficient (D qp ) and the pair-breaking coefficient (η), showing there is a significant increase in η as the bath temperature increases, which can be the reason for the increasing responsivity of TiN under illumination observed elsewhere [13]. The fitted D qp is significantly smaller than expected, which can be why the QPs relax quickly after the photon absorption.…”

“…The TiN films are deposited on sapphire substrates by magnetron sputtering, which are then patterned into resonators using photolithography and reactive ion etching. The films' internal quality factor Q i is relatively low as they are fabricated on sapphire [13,14,39], which is probably due to the lattice mismatch on their interface. The MKIDs design of TiN 1K can be found in the appendix B The design of TiN 2K and TiN 4K can be found in our previous publication [13].…”

“…The films' internal quality factor Q i is relatively low as they are fabricated on sapphire [13,14,39], which is probably due to the lattice mismatch on their interface. The MKIDs design of TiN 1K can be found in the appendix B The design of TiN 2K and TiN 4K can be found in our previous publication [13].…”

“…A direct measurement of the QP diffusion coefficient of TiN at low temperatures is needed to investigate the diffusion coefficient further. The low diffusion coefficient can be the reason for the response of the MKIDs made of disordered superconductors being small [13,14].…”

“…Second, their resonance frequency shift versus T bath is proportional to the optical power (P opt ) under steady illumination [3,11], while the resonance frequency of MKIDs made of superconductor described by BCS theory [12] is proportional to [9] √ P opt . Third, the response in the optical band is much smaller than expected [8,13,14]. Fourth, the density of states (DoS) of the QPs below the energy gap (∆) is not zero due to the energy gap being smeared by the disorder [15].…”