We study microwave superconducting stripline resonators made of NbN on Sapphire substrate. A section in the resonator is made of a narrow (150 nm) and thin (8 nm) meander strip. A continuous wave at frequency close to one of the resonances is injected into the resonator and the reflected power off the resonator is measured. Novel, self-sustained oscillations of the reflected power, at frequencies of up to 60MHz are observed. To the best of our knowledge such oscillations were not reported before in similar systems.
Near the onset of these oscillations the device exhibits a chaotic like behavior and is characterizes by giant nonlinearity. Intermodulation characterization performed in this regime yields extremely high intermodulation gain (about 30dB), which is accompanied by a very strong noise squeezing (about 45dB squeezing factor) and period doubling of various orders. We also study the response of the device to IR (1550 nm wavelength) illumination impinging on the meander strip. To characterize the response time of the system we modulate the impinging optical power with a varying frequency. We observe extremely fast (modulation frequencies of up to 8GHz) and sensitive (optical power below 100 fW) response near the onset of the self-sustained oscillations.
To account for our findings we propose a theoretical model according to which the self-sustained oscillations are originated by thermal instability in the meander strip. A comparison of the model’s predictions with experimental results yields a partial qualitative agreement.