Constant-voltage resonant steps in underdamped Josephson-junction arrays and possibilities for optimal millimeter-wave power output

When a parallel external magnetic field is applied to underdamped Josephson -junction arrays, constant-voltage steps appear in their current-voltage ch aracteristics. These steps correspond to different numbers of rows being sw itched to a new resonant state. If the number of switched rows is larger th en a threshold number, the array radiates coherent microwave radiation. Whe n the array is biased on a step, the number of radiating rows stays fixed a nd we can change the input power, P-DC, by changing the bias current. We me asure the output power, P-AC, as a function of P-DC This dependence is line ar at high powers with a slope alpha, while at low powers P-AC vanishes non linearly with P-DC. For a given array, the slope a is larger for steps that correspond to a larger number of switched rows. We present a systematic st udy of the dependence of the slope a on the size of the array and discuss i ts implications for obtaining optimal DC-to-AC conversion efficiency.