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If driven sufficiently strongly, superconducting microresonators exhibit nonlinear behavior including response bifurcation. This behavior can arise from a variety of physical mechanisms including heating effects, grain boundaries or weak links, vortex penetration, or through the intrinsic nonlinearity of the kinetic inductance. Although microresonators used for photon detection are usually driven fairly hard in order to optimize their sensitivity, most experiments to date have not explored detector performance beyond the onset of bifurcation. Here we present measurements of a lumped-element superconducting microresonator designed for use as a far-infrared detector and operated deep into the nonlinear regime. The 1 GHz resonator was fabricated from a 22 nm thick titanium nitride film with a critical temperature of 2 K and a normal-state resistivity of $100, mu Omega,$cm. We measured the response of the device when illuminated with 6.4 pW optical loading using microwave readout powers that ranged from the low-power, linear regime to 18 dB beyond the onset of bifurcation. Over this entire range, the nonlinear behavior is well described by a nonlinear kinetic inductance. The best noise-equivalent power of $2 times 10^{-16}$ W/Hz$^{1/2}$ at 10 Hz was measured at the highest readout power, and represents a $sim$10 fold improvement compared with operating below the onset of bifurcation.
The superconducting critical temperature (Tc > 15K) of niobium titanium nitride (NbTiN) thin films allows for low-loss circuits up to 1.1 THz, enabling on-chip spectroscopy and multi-pixel imaging with advanced detectors. The drive for large scale de
Thin films of TiN were sputter-deposited onto Si and sapphire wafers with and without SiN buffer layers. The films were fabricated into RF coplanar waveguide resonators, and internal quality factor measurements were taken at millikelvin temperatures
We report on the direct measurement of the electron-phonon relaxation time, {tau}eph, in disordered TiN films. Measured values of {tau}eph are from 5.5 ns to 88 ns in the 4.2 to 1.7 K temperature range and consistent with a T-3 temperature dependence
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, opposi
A new operating regime of the Superconducting Quantum Interference Filter (SQIF) is investigated. The voltage to magnetic field response function, V(H), is determined by a Fraunhofer dependence of the critical current and magnetic flux focusing effec