  Solving coupled nonlinear sine-Gordon equations and Maxwell equations
numerically, we study the electromagnetic and superconducting properties of the
single crystal of high-$T_c$ superconductor $\rm{Bi_2Sr_2CaCu_2O_{8+\delta}}$
with a static magnetic field applied parallel to the $ab$-plane and a dc
current fed in along the c-axis. Cavity resonances of transverse plasma occur
in the intrinsic Josephson junctions with frequencies in terahertz regime. It
is revealed that the electromagnetic wave can transmit from the junctions into
space. The emitted energy counted by the Poynting vector is about
$400\rm{W/cm^2}$. The frequency as well as the energy of emission can be tuned
almost continuously by the current and magnetic field.
