  Continuous, coherent sub-terahertz radiation arises when a dc voltage is
applied across a stack of the many intrinsic Josephson junctions in a
Bi2Sr2CaCu2O8+d single crystal. The active junctions produce an equal number of
I-V characteristic branches. Each branch radiates at a slightly tunable
frequency obeying the ac Josephson relation. The overall output is broadly
tunable and nearly independent of heating effects and internal cavity
frequencies. Amplification by a surrounding external cavity to allow for the
development of a useful high-power source is proposed.
