Radiation detection from Fiske steps in Josephson junctions above 200 GHz

We have measured the Josephson radiation emitted at millimeter-wave frequencies by a long Josephson junction dc-current biased on Fiske steps of the current-voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero-field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in-line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon-assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30-Ω normal-state resistance tunnel junction.