  We report a GaAs/AlAs superlattice parametric oscillator. It was pumped by a
microwave field (power few mW) and produced 3rd harmonic radiation (frequency
near 300 GHz). The nonlinearity of the active superlattice was due to Bragg
reflections of conduction electrons at the superlattice planes. A theory of the
nonlinearity indicates that parametric oscillation should be possible up to
frequencies above 10 THz. The active superlattice may be the object of further
studies of predicted extraordinary nonlinearities for THz fields.
