THz parametric gain in semiconductor superlattices in the absence of   electric domains
  We theoretically show that conditions for THz gain and conditions for formation of destructive electric domains in semiconductor superlattices are fairly different in the case of parametric generation and amplification. Action of an unbiased high-frequency electric field on a superlattice causes a periodic variation of energy and effective mass of miniband electrons. This parametric effect can result in a significant gain at some even harmonic of the pump frequency without formation of electric domains and corruption from pump harmonics. 
Timo Hyart, Natalia V. Alexeeva, Ahti Leppanen, and Kirill N. Alekseev