Abstract Understanding and controlling the nonlinear optical properties and coherent quantum evolution of complex multilevel systems out of equilibrium is essential for the new semiconductor device generation. In this work, we investigate the nonlinear system properties of an unbiased quantum cascade structure by performing two-dimensional THz spectroscopy. We study the time-resolved coherent quantum evolution after it is driven far from equilibrium by strong THz pulses and demonstrate the existence of multiple nonlinear signals originating from the engineered subbands and find the lifetimes of those states to be in the order of 4–8 ps. Moreover, we observe a coherent population exchange among the first four intersubband levels during the relaxation, which have been confirmed with our simulation. We model the experimental results with a time-resolved density matrix based on the master equation in Lindblad form, including both coherent and incoherent transitions between all density matrix elements. This allows us to replicate qualitatively the experimental observations and provides access to their microscopic origin.

中文翻译：

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太赫兹量子级联结构的二维光谱

摘要 理解和控制非平衡的复杂多级系统的非线性光学特性和相干量子演化对于新一代半导体器件至关重要。在这项工作中，我们通过执行二维太赫兹光谱研究了无偏量子级联结构的非线性系统特性。我们研究了被强太赫兹脉冲驱动远离平衡后的时间分辨相干量子演化，并证明了源自工程子带的多个非线性信号的存在，并发现这些状态的寿命约为 4-8 ps . 此外，我们观察到弛豫期间前四个子带间水平之间的相干种群交换，这已通过我们的模拟得到证实。我们使用基于 Lindblad 形式的主方程的时间分辨密度矩阵对实验结果进行建模，包括所有密度矩阵元素之间的相干和非相干转换。这使我们能够定性地复制实验观察并提供对其微观起源的访问。