  We demonstrate theoretically the parametric oscillator behavior of a
two-level quantum system with broken inversion symmetry exposed to a strong
electromagnetic field. A multitude of resonance frequencies and additional
harmonics in the scattered light spectrum as well as altered Rabi frequency are
predicted to be inherent to such systems. In particular, dipole radiation at
the Rabi frequency appears to be possible. Since the Rabi frequency is
controlled by the strength of coupling electromagnetic field, the effect can
serve for the frequency-tuned parametric amplification and generation of
electromagnetic waves. Manifestation of the effect is discussed for III-nitride
quantum dots with strong build-in electric field breaking the inversion
symmetry. Terahertz emission from arrays of such quantum dots is shown to be
experimentally observable.
