Spontaneous Frequency Shift and Phase Delay of Coupled Terahertz Radiation Mediated by the Josephson Plasmon in a Cuprate Superconductor

We examine coupling interactions used to synchronize macroscopic Josephson oscillations induced in intrinsic Josephson-junction (IJJ) mesa stacks made of a $\mathrm{Bi}2212$ single crystal. Synchronized radiations of terahertz electromagnetic (EM) waves are detected under common voltage and current bias operations of two connected mesas with close individual radiation frequencies, while uncoupled and bimodal radiations are frequently observed in two mesas with different individual radiation frequencies. Detailed observations of the polarizations of the EM waves emitted when two mesas are biased in parallel or series allow us to reveal the coupling matrix components, which include ratios of synchronized IJJs in the mesas and phase delay between the macroscopic Josephson oscillations. A frequency evolution of the phase delay implies that the coupling between the Josephson oscillations is mediated by the small amplitude Josephson plasmon inside the superconducting substrate. This finding stimulates systematic survey on polarization of EM wave emitted from synchronized multiple mesa devices in order to realize powerful terahertz emissions from superconductors.