30 THz mixing experiments on high temperature superconducting Josephson junctions

We have investigated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// superconductor-normal-superconductor Josephson junctions as mixers of 30 THz radiation. We have directly observed (2nd order) difference frequencies from 10 MHz to 12.8 GHz between two CO/sub 2/ laser lines. Applying a third microwave signal to the junction, we have observed CO/sub 2/ laser difference frequencies up to 27 GHz. The d.c. bias dependence of the difference frequency signal, as well as other evidence, suggests two distinct mixing mechanisms: hot-electron mixing in the junction banks at high d.c. biases, and bolometric Josephson mixing at low d.c. biases. The latter is the first observation of Josephson mixing at CO/sub 2/ laser frequencies in high-T/sub c/ junctions. The Josephson mixing has generated observable mixing products up to 6th order.>