The ionization of deep impurity centers was observed under radiation in the THz range, where the photon energy was much less than the binding energy of the impurities. It was shown that, for radiation intensities that were not too high, the ionization was caused by the Poole-Frenkel effect. As in the well known case of direct current fields, the electric field of the high-frequency radiation lowered the Coulomb potential barrier and enhanced the thermal emission of carriers.

S.D.Ganichev, B.Mayerhofer, I.N.Yassievich, J.Diener, W.Prettl: Materials Science Forum, 1995, 196-201, 1543-6