  Subterahertz radiation emitted from a variety of short rectangular-, square-,
and disk-shaped mesas of intrinsic Josephson junctions fabricated from a
Bi2Sr2CaCu2O8+d single crystal was studied from the observed I-V
characteristics, far-infrared spectra, and spatial radiation patterns. In all
cases, the radiation frequency satisfies the conditions both for the ac
Josephson effect and for a mesa cavity resonance mode. The integer higher
harmonics observed in all spectra imply that the ac Josephson effect plays the
dominant role in the novel dual-source radiation mechanism.
