  The nano-scale intrinsic Josephson junctions in highly anisotropic cuprate
superconductors have potential for generation of terahertz electromagnetic
waves. When the thickness of a superconductor sample is much smaller than the
wavelength of electromagnetic waves in vacuum, the superconductor renders
itself as a cavity. Unlike conventional lasers, the presence of the cavity does
not guarantee a coherent emission because of the internal degree of freedom of
the superconductivity phase in long junctions. We study the excitation of
terahertz wave by solitons in a stack of intrinsic Josephson junctions,
especially for relatively short junctions. Coherent emission requires a
rectangular configuration of solitons. However such a configuration is unstable
against weak fluctuations, contrarily solitons favor a triangular lattice
corresponding to an out phase oscillation of electromagnetic waves. To utilize
the cavity, we propose to use an array of stacks of short intrinsic Josephson
junctions to generate powerful terahertz electromagnetic waves. The cavity
synchronizes the plasma oscillation in different stacks and the emission
intensity is predicted to be proportional to the number of stacks squared.
