M.Sc Thesis

M.Sc StudentAbdo Baleegh
SubjectNonlinear Effects and Critical Coupling in NbN
Superconducting Microwave Resonators
DepartmentDepartment of Electrical and Computer Engineering
Supervisor ASSOCIATE PROF. Eyal Buks


       In this work, we have designed and fabricated several NbN superconducting stripline  microwave resonators sputtered on sapphire substrates. The low temperature response exhibits strong and unexpected nonlinear effects, including sharp jumps as the frequency or power are varied, frequency hysteresis loops changing direction as the input power is varied, and others. Contrary to some other superconducting resonators, a simple model of a one-dimensional Duffing resonator cannot account for the experimental results.
    Whereas the physical origin of the unusual nonlinear response of our samples remains an open question, our intensive experimental study of these effects under varying conditions provides some important insight. We consider a hypothesis according to which Josephson junctions forming weak links between the grains of the NbN are responsible for the observed behavior. We show that most of the experimental results are qualitatively consistent with such hypothesis.
    While revealing the underlying physics remains an outstanding challenge for future research, the utilization of the unusual nonlinear response for some novel applications is already demonstrated in the present work. In particular we operate the resonator as an intermodulation amplifier and find that the gain can be as high as 15 dB. To the best of our knowledge, intermodulation gain greater than unity has not been reported before in the scientific literature. In another application we demonstrate for the first time that the coupling between the resonator and its feed line can be made amplitude dependent. This novel mechanism allows us to tune the resonator into critical coupling conditions.