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

在室温半导体器件中提供移动的电荷的浅掺杂杂质在低温下扮演着新的角色，多余的电荷以一组类似于气相原子的状态束缚在它们的母离子上。除了它们在量子信息技术中的潜力，这种“准原子”还可以为研究强烈的光-物质相互作用提供一个独特的平台，由于与主晶格的强相互作用，杂质态的光子能量尺度从紫外（对于气体原子）下降到太赫兹（THz）范围，结合场为kV/cm，空间范围为几纳米。 此外，杂质状态依赖于掺杂剂和宿主物种，导致整个准原子家族的轨道和自旋密度偏离简单的氢模型。 在固体物理中，基质相互作用也是一个关键问题，例如声子相互作用对激发态的寿命和退相有着重要的影响，本文利用强THz脉冲实验研究了传统半导体基质中施主/受主杂质的非线性响应可以观察到两个关键现象：（一）动态斯塔克（Autler-Townes）效应和态间量子干涉，和（ii）高次谐波产生（HHG）随后的隧道电离由于随后的加速载流子在太赫兹领域。在这里，我们采用了一个互补的方法与单周期太赫兹脉冲（在我们的实验室）和多周期太赫兹脉冲（在自由电子激光设施）。 多周期脉冲允许更常规的状态选择性激发和可以在场修饰（Floquet）状态方面描述的系统，而具有单周期脉冲的宽带激发允许直接在时域中测量超快演化并探测传播效应。该实验还进一步将太赫兹范围内的四波混频泵浦-探测方法应用于量子系统的研究，这是最近才通过飞秒激光系统实现的。虽然从这些杂质中观察到的HHG起源于束缚态的电离，并且发射是由于载流子的随后加速，类似于对气相原子进行深入研究的情况，但加速发生在相邻的半导体带中，根据最近对具有强烈中红外激发的大块半导体中的HHG的研究。因此，我们的实验结果将进一步统一这两个现有的领域。

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.