Excitation of deep defects by intense terahertz radiation

An analysis is done of the ionization of deep impurity centers by high-inte nsity terahertz radiation, with photon energies tens of times lower than th e impurity ionization energy. Under these conditions, ionization can be des cribed as direct tunneling and phonon-assisted tunneling in which carrier e mission is accompanied by defect tunneling in configuration space and elect ron tunneling in the electric field of the radiation. Within a broad range of intensity, frequency, and temperature, the terahertz electric field of t he radiation acts like a static field. For very high frequencies and low te mperatures an enhancement of tunneling as compared to static fields was obs erved. The transition between the quasi-static and the high frequency regim e is determined by the tunneling time. For the case of deep impurities this is the time of redistribution of the defect vibrational system which depen ds strongly on temperature and the impurity structure.