Response of Nb-aSi-Nb junctions to 604 GHz radiation

We have measured the response of Nb-aSi-Nb junctions to 604 GHz radiation from an optically pumped far-infrared (FIR) laser source. These rugged and thermally cyclable junctions situated at the midpoint of 180 μm long dipole antennas, were fabricated on oxidized silicon wafers using the Selective Niobium Anodization Process (SNAP). Josephson current densities were\sim 9,000A/cm², the McCumber parameters βcwere ∼3.5, and the Josephson plasma frequencies\omega_{J}\sim2.5\times10^{12}. On a junction with a normal state resistance of\sim7\Omega, we observed three Josephson steps and one photon-assisted tunneling step. The widths of the Josephson steps were studied as a function of the laser power. An RSJ model computer simulation with a nonlinear quasiparticle conductance and an rf current bias (assumed because of the low junction resistance) is able to account reasonably well for the laser-power dependence of the critical current (zeroth step) and the second step. However, the temperature dependence is more complex, and a discrepancy still exists between the RSJ model calculation and the data for the hysteretic first step.