Electron dynamics in semiconductor quantum structures investigated by time-resolved terahertz spectrosccopy

半导体量子结构中的电子动力学知识对于实现工作在太赫兹（THz）频率范围内的超高速电子器件是必不可少的。由于超快电子运动伴随着电磁波辐射，因此可以通过测量时域辐射（时间分辨太赫兹波谱）来研究电子动态运动的信息。我们利用这种技术研究了电子动力学和运动的相干性。本项目取得的主要成果有：(1)当两个量子阱以量子力学耦合时，电子通过隧穿效应（“量子拍”）在两个量子阱之间相干地来回运动。我们成功地探测到由电子量子拍引起的振荡太赫兹辐射，并确定了在低温下相干隧穿的衰减时间为6ps。(2)研究了选择性掺杂量子阱中二维自由载流子等离子体振荡的动力学。发现二维等离子体激元有两种不同的激发机制；相干脉冲受激拉曼过程和光激发载流子的非相干突然加热。(3)利用相关测量技术，建立了20太赫兹带宽、50 fs时间分辨率的太赫兹探测系统。利用该系统，我们研究了砷化镓耗尽区的非平衡电子输运。发现在加速后的700fs内，电子处于速度超调模式，光激发电子相干地激发光声子。

Knowledge on the electron dynamics in semiconductor quantum structures is indispensable for realization of ultrahigh speed electron devices which operate in the terahertz (THz) frequency range. Since ultrafast electron motions accompany radiation of electromagnetic waves, information on the dynamical motion of electrons can be investigated by measuring the radiation in the time domain (time-resolved THZ spectroscopy). We have investigated the electron dynamics and coherence of motions by using this technique. The major results obtained in this projects are ;(1) When two quantum wells are coupled quantum mechanically, electron moves back and forth coherently between the two quantum wells via tunneling effect ("quantum beat"). We have successfully detected the oscillatory THz radiation due to the quantum beats of electrons and determined the decay time of the coherent tunneling to be 6 ps at low temperatures.(2) We have investigated the dynamics of two-dimensional free carrier plasma oscillations in selectively doped quantum wells. It is found that there are two distinctive excitation mechanisms of the 2D plasmons; coherent impulsive stimulated Raman process and incoherent sudden heating by photoexcited carriers.(3) We have built up a THz detection system with 20 THz bandwidth and 50 fs time resolution by using a correlation measurement technique. By using this system, we have investigated the nonequilibrium electron transport in GaAs depletion regions. It is found that within 700 fs after the acceleration electrons are in a velocity overshoot mode and that photoexcited electrons coherently excite optical phonons.