Terahertz electromagnetic wave emission by using intrinsic Josephson junctions of high-Tc superconductors

High-Tc cuprate superconductors like Bi₂Sr₂CaCu₂O_8+y (Bi-2212) show very large anisotropy in the electrical conduction. Inside the CuO₂ plan the conduction looks like that in good metals, while the block layers located between CuO₂ planes behave like insulating barriers. Thus, neighboring CuO₂ layers are linked by the Josephson coupling in superconducting states, and the Bi-2212 single crystal can be regarded as a natural serial array of Josephson junctions along the c-axis. It has been found out that in such a system a unique excitation mode called the collective Josephson plasma exists. The mode collectively oscillates along the c-axis, while it propagates along any direction ranged from the ab-plane to the c-axis. If the mode can be coherently excited, the system should be very useful as a device generating strong electromagnetic wave emission. In this paper we review a theoretical framework describing the Josephson plasma modes. The Josephson vortex penetrates into the sample under the magnetic field parallel to the CuO₂ plane and it moves along the ab-plane under an external current parallel to the c-axis. In the vortex flow state, the vortex speed can reach and exceed the propagating velocities of the Josephson plasma modes due to very weak dissipation. Then, the vortices strongly couple with the plasma modes and change their flowing lattice configurations into patterns dependent on the interacting plasma mode profiles. Among many expected flow patterns, we pay attention to a special state resonating with a plasma mode in-phase along the c-axis because all junctions synchronize in such a state. Large-scale numerical simulations by Machida et al. confirmed that the special state characterized by the rectangular lattice stably appears in a wide range of I–V characteristics. In this paper, we report their recent simulation results.