Chaotic behavior observed in a dc-biased Josephson junction driven at FIR frequencies

The response of a Nb-aSi-Nb junction to far-infrared laser radiation at 245, 419, and 604 GHz. At 419 GHz, which is close to the junction plasma frequency, the laser-induced steps in the I-V curves exhibit chaotic behavior over a considerable range of laser driving power. For example, regions of meandering voltage may appear on an otherwise well-defined and flat step. At such chaotic parts of the I-V curves, there is an extremely high level of low-frequency noise, corresponding to a noise temperature of about 100 billion K at frequencies around 100 Hz. Negative resistance regions are also observed. At some laser power levels, the I-V curve shows the 2/3 Josephson subharmonic step even in the absence of the first step. Smooth I-V curves reappear as the laser power is increased, showing the existence of chaos-free windows.