Coherent radiation from active Josephson antennas

We performed spectral measurements of two low-temperature discrete Josephson junction (JJ) arrays on a Fourier-transform spectrometer. The arrays contain up to 9000 niobium based junctions located in the area 55 mm and composed of different designs. The spectrum of each array demonstrates a narrow peak in the frequency range near 140 GHz with the line width 160170 MHz corresponding to the resolution limit of the spectrometer. It implies the coherent subterahertz radiation from niobium JJ arrays that is necessary for most applications where the JJ array based on similar technology would be implemented. The radiation coherence follows from numerical simulation of Josephson antennas with the self-made algorithm combining the Finite-Difference Time Domain Method with the the Resistively and Capacitively Shunted Junction (RCSJ) model. Exploring the spectrum of Beverage- like Josephson antenna simulated by this algorithm we revealed the pure monochromatic emission of all junctions.