Nonlinear dynamics of impurity states in semiconductors driven by intense THz pulses

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Shallow dopant impurities, which provide the mobile charges in room-temperature semiconductors devices, take on a new role at low temperature, where the excess charges are instead bound to their parent ions with a set of states analogous to gas-phase atoms.Besides their potential for quantum information technology, such “quasi-atoms” can offer a unique platform for studying intense light-matter interactions, bridging the gap between high-field ultrafast phenomena in free atoms and bulk semiconductors.Due to the strong interaction with the host lattice, the photon energy scale of the impurity states is shifted from the ultraviolet (for gas atoms) down to the terahertz (THz) range, with binding fields of kV/cm and spatial extent of several nanometers. Moreover, the impurity states depend on both dopant and host species, resulting in a whole family of quasi-atoms with orbital and spin densities which deviate from a simple hydrogen model. The host interactions also confront key aspects in solid-state physics, such as the effect of phonon interactions, which have a profound effect on the lifetimes and dephasing of the excited states.Here we employ experiments with intense THz pulses to study of the non-linear response of donor/acceptor impurities in conventional semiconductor hosts (Si, Ge, GaAs).Two key phenomena can be observed: (i) the dynamic Stark (Autler-Townes) effect and quantum interference between states, and (ii) higher-harmonic generation (HHG) following tunnel ionization due to the subsequent acceleration of carriers in the THz field.Here we employ a complementary approach with both single-cycle THz pulses (in our laboratory) and multi-cycle THz pulses (in free-electron-laser facilities). The multi-cycle pulses allow a more conventional state-selective excitation and a system that can be described in terms of field-dressed (Floquet) states, whereas broadband excitation with single-cycle pulses allows one to measure the ultrafast evolution directly in the time domain and to probe for propagation effects. It also opens the possibility to control the resulting quantum state via pulse shaping.The experiments also further the application of four-wave-mixing pump-probe methodology in the THz range to the study of quantum systems, which has only recently become accessible with femtosecond laser systems. While the HHG observed from such impurities has its origin in the ionization of bound states and the emission is due to the subsequent acceleration of the carriers, analogous to the situation studied intensely for gas-phase atoms, the acceleration takes place in the adjacent semiconductor band(s), as per recent studies of HHG in bulk semiconductors with intense mid-infrared excitation. Our experimental findings will hence further unify these two existing fields.