Nonlinear spectroscopy of impurity states in semiconductors driven by intense THz fields

We present an experimental investigation of the nonlinear response of acceptor impurities in semiconductors to coherent excitation with intense THz fields. In Zn-doped GaAs, we observed a well-defined saturation of the transition from the impurity ground state to the excited states and valence band. In B-doped Si, however, there is a clear indication of field-induced distortion of the impurity energy states: in the highly B-doped sample, an absorption band due to boron-carbon pairs (boron-X center) emerges and shows strong field-induced broadening and splitting. Similar effects were also observed in the substitutional boron impurity states at lower doping density.