Stimulated emission and amplification in Josephson junction arrays

We report measurements of mm-wavelength radiation from two-dimensional arra ys of underdamped Josephson junctions. All of our samples emit coherently i n a novel synchronized state, which is triggered by a resonance in the arra y structure. Measurements of the detected power as a function of the number N of active junctions show a threshold, suggesting population inversion. A bove threshold, the power scales with N-2 up to an array size bigger than t he free-space radiation wavelength. The highest measured conversion efficie ncy from de to ac power is about 17%. Our data are consistent with stimulat ed emission causing coherence and cannot be explained by existing classical coupling mechanisms.