We have studied the response of a YBCO/PBCO/YBCO ramp-type junction to coherent radiation at 176 and 270 GHz. The I-V characteristic of the junction closely resembles the prediction of the RSJ model. The I(c)R(n) product of the junction is 0.25 mV at 5 K. The millimeter-wave radiation is coupled to the junction via a quasioptical structure that focuses the radiation onto the junction through a yttrium-stabilized ZrO2 substrate. At 176 GHz, we have observed as many as six Shapiro steps at the maximum power level of our Gunn oscillator-pumped frequency doubler. Shapiro steps are still clearly seen up to 65 K. The amplitudes of the zeroth, first, and second Shapiro steps, as functions of the square root of the radiation power, agree remarkably well with a Bessel function fit, indicating the junction is voltage-biased at the radiation frequency (rf). At 270 GHz, due to a combination of the heavy rf loss in the ZrO2 substrate and the lack of radiation power, we have observed only the first Shapiro step.
