Theory of coherent collective excitations in semiconductor superlattices excited by ultrashort optical pulses

我们研究了超短脉冲激光激发半导体超晶格（多量子威尔斯：MQW）中的相干振荡，并研究了与集体激发相关的太赫兹（THz）电磁波（包括激光）的产生机制。我们的研究结果概括如下：（1）我们研究了超晶格中沿生长轴沿着传播的小带内相干等离子体激元。为了将我们的结果与观测到的太赫兹辐射以及反射电光采样数据进行比较，我们构建了一个基于密度矩阵的介电响应理论。利用Kronig-Penney模型，通过改变阱宽与势垒宽的比值，计算了等离子体激元的色散关系。我们的结果表明，在多个微带的等离子体频率作为电子数的函数的离散性，并对应于观察到的太赫兹辐射从AlGaAs/GaAs超晶格的出现和消失。(2)We模拟了高电场下GaAs p-i-n二极管中相干LO声子的THz辐射以及非平衡光激发载流子的超快运动。因此，我们揭示了LO声子与载流子过冲之间的同步对于产生相干LO声子振荡的重要性，并预测了在150 kV/cm以上的极强电场下THz辐射强度的饱和行为。(3)We建立了非对称耦合双量子威尔斯（ACDQWs）中太赫兹拉曼激光的理论。在这种结构中，由于集体子带间等离子体的受激斯托克斯拉曼散射产生适当的激光增益。此外，还揭示了相干子带间等离子体激元与LO声子的相互作用对确定激光频率起着关键作用。我们的结果很好地解释了在AlGaAs/GaAs ACDQWs中观察到的激光频率。

We have explored coherent oscillations in semiconductor superlattices (multiple quantum wells : MQWs) excited by ultrashort pulse lasers, and have studied generation mechanism of electromagnetic waves (including a laser) in a terahertz (THz) regime associated with the collective excitations. Our results are summarized as follows :(1)We have explored the coherent intra-miniband plasmon in a superlattice, which propagates along the growth axis. To compare our results with the observed THz radiation as well as reflective electro-optical sampling data, we have constructed a dielectric response theory based on density matrix. Using Kronig-Penney model, and changing ratio of well width to barrier width, we evaluated the plasmon dispersion relation. Our results indicate the discrete nature of plasma frequency as a function of electron number in multi-minibands, and correspond well to the observed appearance and disappearance of THz radiation from AlGaAs/GaAs superlattices.(2)We simulated THz radiation by coherent LO phonons as well as ultrafast movements of non-equilibrium photoexcited carriers in GaAs p-i-n diodes under high electric field. So we revealed an importance of synchronization between LO phonon and overshoot of carriers for generation of coherent LO phonon oscillation, and have predicted a saturation behavior of THz radiation intensity under extremely strong electric fields above 150 kV/cm.(3)We have constructed a theory of THz Raman laser excited optically in asymmetric coupled double quantum wells (ACDQWs). A stimulated Stokes Raman scattering due to collective inter-subband plasm on produces the proper laser gain in this structure. Further, it was revealed that the interaction of coherent intersubband plasmon with LO phonons plays the key role for a determination of the laser frequencies. Our results explain well the observed laser frequency in AlGaAs/GaAs ACDQWs.