Optical Properties of Terahertz-Modulated Quantum Structures

本研究基金主要研究半导体异质结构在太赫兹电磁场作用下的光学特性，包括量子阱（QW’s）、量子线（QWR’s）、量子点（QD’s）和微腔（MC’s）。这一研究领域除了具有根本性的意义外，还受到高速电子学和高带宽光通信的双重推动。本研究特别感兴趣的是低载流子密度，其中很少有电子或空穴被光束激发；载流子的相干特性起着至关重要的作用。先前的研究考虑了超快光脉冲在太赫兹场存在下激发的电子-空穴对动力学。由于亚ps光脉冲的带宽可以超过10 meV，因此具有不同程度多余能量（或其内部运动不同状态的激子）的e-h对被形成波包的光脉冲激发。这些研究提供了一些原子物理现象的类比，包括里德伯波包的动力学、阈值以上电离和高场谐波的产生。所提出的工作的重点是进一步探索这种类比，但更重要的是要进入固有的固态效应领域，如多体效应和载流子-声子散射，通过导致光学激发的e-h对的减相而开始使它们的存在感到。因此，第一个光学非线性，踢在光强度增加超过线性光学体制将被研究。具体来说，载流子-载流子和载流子-声子散射如何导致光激发的电子激发的失相，以及这种失相如何改变由短光脉冲激发并由太赫兹场驱动的e-h波包的空间运动？这种波包发出的声子的性质是什么？太赫兹驱动的e-h波包能否发射声子的相干波包？散射率本身能被太赫兹驱动的e-h波包的动力学改变吗？除了在半导体光谱学中是一个尚未开发的重要基础领域外，光通过太赫兹调制量子阱的传播为一类时域单粒子量子输运现象提供了光学模拟，否则这些现象是不可行的。特别是，在隧穿过程中跟踪相干波包的时间演变是一个基本的兴趣，但在量子输运实验中，即使不是在实践中不可能，也是困难的。幸运的是，在标量经典电磁波方程和单粒子薛定谔方程之间有一个密切的类比。这意味着，在一定的状态下，人们可以通过适当的光传播实验，通过对应来模拟粒子的量子力学动力学。这种对应关系延伸到相位，即量子力学相位映射到光学相位。显然，干涉实验在光学中是常规的，但在量子输运中需要力作。一类特别有趣的现象涉及通过时间调制电位的量子隧穿。与太赫兹照明半导体光谱上太赫兹边带形成相关的同类型过程与量子输运现象之间的联系。理论研究将集中在量子阱/原子或光学/输运类比都不完全成立的情况下，例如非线性光学情况，其中太赫兹调制的半导体异质结构提供了新的可能性。具体来说，将研究连续波和超快光脉冲在脉冲或窄带太赫兹场作用下通过异质结构的传播。%%%本研究资助重点研究半导体异质结构，包括量子阱（QW’s）、量子线（QWR’s）、量子点（QD’s）和微腔（MC’s）在太赫兹（THz）电磁场存在下的光学特性。这一研究领域除了具有根本性的意义外，还受到高速电子学和高带宽光通信的双重推动。本研究特别感兴趣的是低载流子密度，其中很少有电子或空穴被光束激发；载流子的相干特性起着至关重要的作用。＊＊＊

0073364CitrinThis research grant focuses on the optical properties of semiconductor heterostructures, including quantum wells (QW's), quantum wires (QWR's), quantum dots (QD's) and microcavities (MC's), in the presence of terahertz (THz) electromagnetic fields. This research area, in addition to being of fundamental interest, has received a two-fold impetus from high-speed electronics and from high-bandwidth optical communications. Of particular interest to this research is the regime of low carrier density in which few electrons or holes are excited by the optical beam; the coherence properties of the carriers play an essential role. Previous reseach considered the dynamics of electron-hole (e-h) pairs excited by ultrafast optical pulses in the presence of THz fields. Because the bandwidth of a sub-ps optical pulse can be in excess of 10 meV, e-h pairs with varying degrees of excess energy (or excitons in different states of their internal motion) are excited by the optical pulse forming a wavepacket. Such studies provide an analogy to several atomic physics phenomena, including the dynamics of Rydberg wavepackets, above-threshold ionization, and high-field harmonic generation. The focus of the proposed work is to explore this analogy further, but more so to push into the domain where intrinsic solid state effects, such as many-body effects and carrier-phonon scattering, begin to make their presence felt by leading to dephasing of the optically excited e-h pairs. Thus, the first optical nonlinearities that kick in as the optical intensity is increased beyond the linear optical regime will be studied. Specifically, how do carrier-carrier and carrier-phonon scattering lead to dephasing of the optically excited electronic excitations, and how does this dephasing modify the spatial motion of e-h wavepackets excited by short optical pulses and driven by THz fields? What is the nature of phonons emitted by such wavepackets; can coherent wavepackets of phonons be launched by THz-driven e-h wavepackets? Can scattering rates themselves be modified by the dynamics of the THz-driven e-h wavepackets?In addition to being an untapped area of fundamental importance in the spectroscopy of semiconductors, light propagation through THz-modulated quantum wells povides an optical analog for a class of time-domain single-particle quantum tranport phenomena that are otherwise infeasible to study. In particular, tracking the temporal evolution of coherent wavepackets during the tunneling process is of fundamental interest and yet difficult, if not in practice impossible, to access in quantum transport experiments. Fortuitously, there is a close analogy between the scalar classical electromagnetic wave equation and the single-particle Schroedinger equation. This means that in a certain regime one can model by correspondence the quantum mechanical dynamics of a particle by an appropriate light-propagation experiment. This correspondence extends to the phase, i.e., quantum mechanical phase maps to optical phase. Clearly, interferometric experiments are routine in optics but require a tour de force effort in quantum transport. A particularly interesting class of phenomena involves quantum tunneling through a time-modulated potential. The connections between the same type of processes that are associated with the formation of THz sidebands on optical spectra of THz illuminated semiconductors with quantum transport phenomena.The theoretical research will focus on regimes where neither the QW/atom or optical/transport analogies entirely hold, such as the nonlinear optical regime - where THz-modulated semiconductor heterostructures present new possibilities. Specifically, the propagation of cw and ultrafast optical pulses through heterostructures subjected to pulsed or narrow-band THz fields will be studied.%%% This research grant focuses on the optical properties of semiconductor heterostructures, including quantum wells (QW's), quantum wires (QWR's), quantum dots (QD's) and microcavities (MC's), in the presence of terahertz (THz) electromagnetic fields. This research area, in addition to being of fundamental interest, has received a two-fold impetus from high-speed electronics and from high-bandwidth optical communications. Of particular interest to this research is the regime of low carrier density in which few electrons or holes are excited by the optical beam; the coherence properties of the carriers play an essential role. ***