Terahertz emission from mutually synchronized standalone Bi2Sr2CaCu2O8+x intrinsic-Josephson-junction stacks

Suitably patterned single crystals made of the cuprate superconductor Bi2Sr2CaCu2O8+x (BSCCO), intrinsically forming a stack of Josephson junctions, can generate electromagnetic radiation in the lower terahertz regime. Because of Joule heating, the emission power of single stacks seems to be limited to values below 100 mu W. To increase the radiation power, mutually synchronized arrays situated on the same BSCCO base crystal have been studied. A maximum power of almost 1 mW has been achieved by synchronization of three stacks. Mutual electromagnetic interactions via a connecting BSCCO base crystal have been considered essential for synchronization, but the approach still suffers from Joule heating, preventing the synchronization of more than three stacks. In the present paper we show, on the basis of two emitting stacks, that mutual synchronization can also be achieved by standalone stacks contacted by gold layers and sharing only a common gold layer. Compared with BSCCO base crystals, the gold layers have a much higher thermal conductivity and their patterning is not very problematic. We analyze our results in detail, showing that the two oscillators exhibit phase correlations over a range of +/- 0.4 GHz relative to their center frequencies, which we studied mainly between 745 and 765 GHz. However, we also find that strong phase gradients in the beams radiated from both the mutually locked stacks and the unlocked stacks play an important role and, presumably, diminish the detected emission power due to destructive interference. We speculate that the effect arises from higher-order cavity modes that are excited in the individual stacks. Our main message is that the mutual interaction provided by a common gold layer may open new possibilities for relaxing the Joule-heating problem, allowing the synchronization of a higher number of stacks. The approach may also allow one to synchronize several stacks that are comparatively small in size and less prone to the strong phase gradients we observed. Our findings may boost attempts to substantially increase the output power levels of BSCCO terahertz oscillators.