  We report on the successful terahertz emission (0.6$\sim$1 THz) that is
continuous and tunable in its frequency and power, by driving Josephson
vortices in resonance with the collective standing Josephson plasma modes
excited in stacked Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ intrinsic Josephson junctions.
Shapiro-step detection was employed to confirm the terahertz-wave emission. Our
results provide a strong feasibility of developing long-sought solid-state
terahertz-wave emission devices.
