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

We have investigated 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 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 1st step. Smooth I-V curves reappear as the laser power is increased, showing the existence of chaos-free windows. The I-V curves taken at 245 GHz also show chaotic behavior, but those taken at 604 GHz are quite smooth. All these features are in good agreement with the results of digital simulations which confirm their chaotic nature.