Operation of a titanium nitride superconducting microresonator detector in the nonlinear regime

TL;DR

This study explores the nonlinear behavior of a titanium nitride superconducting microresonator used as a far-infrared detector, demonstrating improved sensitivity when operated deep into the nonlinear regime beyond bifurcation.

Contribution

It provides the first detailed measurements of a superconducting microresonator's performance deep into the nonlinear regime, showing enhanced sensitivity and noise performance.

Findings

Nonlinear kinetic inductance describes the device behavior.

Operating beyond bifurcation improves noise-equivalent power.

Deep nonlinear operation enhances detector sensitivity.

Abstract

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.