30 THZ MIXING EXPERIMENTS ON HIGH-TEMPERATURE SUPERCONDUCTING JOSEPHSON-JUNCTIONS

We have investigated YBa2Cu3O7-delta superconductor-normal-superconduc tor Josephson junctions as mixers of 30 THz radiation. We have directe d observed (2nd order) difference frequencies from 10 MHz to 12.8 GHz between two CO2 laser lines. Applying a third microwave signal to the junction, we have observed CO2 laser difference frequencies up to 27 G Hz. The d.c. bias dependence of the difference frequency signal, as we ll as other evidence, suggests two distinct mixing mechanisms: hot-ele ctron mixing in the junction banks at high d.c. biases, and bolometric Josephson mixing at low d.c. biases. The latter is the first observat ion of Josephson mixing at CO2 laser frequencies in high-T-c junctions . The Josephson mixing has generated observable mixing products up to 6th order.