All-optical generation and coherent control of ultrafast shift and injection currents in GaAs quantum well structures

利用飞秒激光激发在半导体中产生电流的纯光学方法，即无偏置电场，不仅成为研究非线性相干光-物质相互作用的有力手段，而且为产生太赫兹电磁波开辟了广阔的前景。不同的物理过程，如光学整流、移位电流产生和注入电流产生，都被认为是光学感应电流的来源。所有这些效应都代表了相干的二阶非线性光学相互作用。虽然在体相GaAs中，注入电流的产生是对称禁止的，但我们最近已经证明，在特别设计的，即(110)取向的情况下，可以产生注入电流。更令人惊讶和意想不到的是，我们的发现是，注入电流可以测量到最低激子共振的共振激发。该方案的总体目标是从实验和理论上分析共振激子激发产生不同类型注入电流的条件。我们将基于扩展的半导体Bloch方程，对迄今尚未确定的潜在物理过程进行合理的微观理论描述和分析。我们的共同研究将为控制半导体纳米结构中的非线性光-物质相互作用和全光电流产生的基本机制提供新的见解。

Generating electrical currents in semiconductors by purely optical means, i.e., without electrical bias fields, using femtosecond laser excitation has not only become a powerful means for studying nonlinear coherent light-matter interaction but also opens promising options for THz electromagnetic wave generation. Different physical processes, like optical rectification, shift current generation, and injection current generation, have been identified as the sources for optically-induced currents. All these effects represent coherent second-order nonlinear optical interactions. While the generation of injection currents is symmetry forbidden in bulk GaAs, we have recently demonstrated that in especially designed, i.e., (110)-oriented, GaAs/AlGaAs quantum well structures injection currents can be generated. Even more surprising and unexpected was our finding that injection currents could be measured for resonant excitation of the lowest exciton resonances. The overall goal of this proposal is to analyze experimentally and theoretically the conditions under which the generation of different types of injection currents by resonant exciton excitation is possible. A sound microscopic theoretical description of the so far not identified underlying physical processes, that is based on extended semiconductor Bloch equations, will be developed and analyzed. Our joint investigations will provide new insight in the fundamental mechanisms governing nonlinear light-matter interactions and all-optical current generation in semiconductor nanostructures.