  We examine coupling interactions used to synchronize macroscopic Josephson
oscillations induced in intrinsic Josephson junction (IJJ) mesa stacks made of
a Bi2212 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.
