Use of Dynamical Undulator Mechanism to Produce Short Wavelength   Radiation in Volume FEL (VFEL)
  VFEL lasing in system with dynamical undulator is described. In this system radiation of long wavelength creates the undulator for lasing on shorter wavelength. Two diffraction gratings with different spatial periods form VFEL resonator. The grating with longer period pumps the resonator with long wavelength radiation to provide necessary amplitude of undulator field. The grating with shorter period makes mode selection for short wavelength radiation. Lasing of such a system in terahertz frequency range is discussed. 
V.G. Baryshevsky, K.G. Batrakov