A theoretical and experimental study is made of a new nonlinear optical effect of tunneling ionization of deep impurity centers in the alternating field of high-intensity far-infrared laser radiation, with photon energies tens of times lower than the impurity ionization energy. The ionization mechanism has been investigated in great detail [1-7] yielding that deep impurities can be ionized by tunneling through the oscillating potential well formed by a strong electric field of FIR radiation together with the attractive potential of the defect.
It is shown that in a certain limit FIR radiation may act like a strong dc electric field ionizing deep impurities in spite of the fact that the field is alternating and the quantum energy is much less than the impurity binding energy. In this quasi-static limit the ionization probability does not depend on the radiation frequency . Carrier emission is accomplished by defect tunneling in configuration space and electron tunneling through the potential well formed by the attractive force of the impurity and the radiation electric field. This phonon-assisted tunneling proceeds at high field strengths into direct tunneling without involving phonons . At low intensities ionization of charged impurites may also occur through the Poole-Frenkel effect [9,10].
Raising the frequency leads to a drastical enhancement of tunneling ionization and the tunneling probability gets frequency dependent . The transition from the frequency independent quasi-static limit to frequency dependent tunneling takes place at wt = 1 where w is the radiation frequency and t is the tunneling time  which is in the case of phonon asissted tunneling controlled by the temperature. This transition to the high-frequency limit represents the boundary between semiclassical physics, where the radiation field has a classical amplitude, and full quantum mechanics where the radiation field is quantized and impurity ionization is caused by multi-photon processes.
In both the quasi-static and the high frequency limit, the application of an external magnetic field perpendicular to the electric field reduces the ionization probability when the cyclotron frequency becomes larger than the reciprocal tunneling time and also shifts the boundary between the quasi-static and the high frequency limit to higher frequencies .
The discovery of tunneling ionization of deep impurities by contactless application of a strong uniform electric field using short FIR laser pulses disclosed a new method for probing deep centers in semiconductors. The dependence of the ionization probability on the electric field strength permits to determine defect tunneling times , the structure of the adiabatic potentials of the defect, the Huang-Rhys parameters of electron-phonon interaction, and the trapping kinetics of nonequilibrium carriers [10,11,13].
Our measurements have been carried out with deep impurities in semiconductors, however, because tunneling is crucial in numerous processes in physics, chemistry, and biology we expect that an enhancement of tunneling by contactless application of coherent radiation will have significant consequences.
 S. D. Ganichev, W. Prettl, and P. G. Huggard, Phys. Rev. Lett. 71, 3882
 S.D. Ganichev, J. Diener, I.N. Yassievich, W. Prettl, B.K. Meyer, and K.W.
Benz, Phys. Rev. Lett. 75, 1590 (1995). (PDF)
 S.D. Ganichev, E. Ziemann, Th. Gleim, W. Prettl, I.N. Yassievich, V.I.
Perel, I. Wilke, and E.E. Haller, Phys. Rev. Lett. 80, 2409 (1998). (PDF)
 S. D. Ganichev, I. N. Yassievich, and W. Prettl, Semicond. Sci. Technol. 11, 679 (1996) (PDF).
 S.D. Ganichev, W. Prettl, and I.N Yassievich, Phys. Solid. State 39, 1703
 S. D. Ganichev, Physica B 273-274, 737 (1999).(PDF)
 S.D. Ganichev, I.N Yassievich, V.I. Perel, H. Ketterl, and W. Prettl, Phys.
Rev. B65, 085203 (2002).(PDF)
 S. D. Ganichev, J. Diener, and W. Prettl, Solid State Commun. 92, 883 (1994).
 S. D. Ganichev, J. Diener, I. N. Yassievich, and W. Prettl, Europhys. Lett.
29, 315 (1995). (PDF)
 S.D. Ganichev, E. Ziemann, W. Prettl, I.N. Yassievich, A.A. Istratov,
and E.R. Weber, Phys. Rev. B61, 10361 (2000).(PDF)
 E. Ziemann, S.D. Ganichev, I.N. Yassievich, V.I. Perel, and W. Prettl,
J. Appl. Phys. 87, 3843 (2000).(PDF)
 A. S. Moskalenko, S. D. Ganichev, V. I. Perel, and I. N. Yassievich, Physica
B 273-274, 1007 (1999).(PDF)
 S. D. Ganichev, I. N. Yassievich, W. Raab, E. Zepezauer, and W. Prettl,
Phys. Rev. B (Rapid Comm.) 55, R9243 (1997).(PDF)
 H. Ketterl, E. Ziemann, S. D. Ganichev, A. Belyaev, S. Schmult, and W.
Prettl, Physica B 273-274, 766 (1999).(PDF)