II-VI族半导体量子点电子自旋、原子核自旋的光学调控

Spin state in semiconductor quantum dots (QDs) is one of the promising candidates for solid-state quantum information processing. In the present study of QD spin manipulation, the used materials are mainly MBE-grown III-V semiconductors such as GaAs and InAs QDs, in which spin manipulation experiments are normally done at low temperatures of liquid helium. However, practical apparatus applications prefer a room-temperature working environment. In our recent studies, long-lived spin coherence with dephasing time of 3.6 ns has been found at room temperature in colloidal CdS QDs. In this project, we will make further studies on optical manipulations of electron and nuclear spin states in CdS and some other II-VI semiconductor QDs. Time-resolved Faraday-rotation and photoluminescence spectroscopy techniques will be used to investigate room-temperature electron-spin transverse/longitudinal relaxation dynamics and its control, as well as nuclear-spin excitation and relaxation process. Spin states in QDs of various dot sizes, various components and surface states will be manipulated by varying the time-frequency domain, power, and polarization of light, as well as the magnetic field intensity in the transverse or longitudinal direction. The involved spin physics and its potential applications will be explored in depth. This project will not only be significative in basic scientific research, but also have an important application background in solid-state quantum information processing at room temperature.

半导体量子点的自旋态是实现固态量子信息处理的理想候选体系之一。目前自旋研究所用的量子点材料主要是利用 MBE 设备生长的GaAs、InAs等III-V 族材料，其中的自旋调控实验通常只能在液氦低温条件下进行。实际器件化的需要希望有个室温的工作环境。我们的前期研究发现胶体CdS量子点系综在室温条件下自旋退相干时间长达3.6 ns。本项目将进一步深入研究以CdS胶体量子点为代表的II-VI族半导体体系的电子自旋、原子核自旋的光学调控，利用时间分辨法拉第旋转光谱、时间分辨荧光方法来研究室温下电子自旋的横向/纵向弛豫动力学及其控制、核自旋的激发与弛豫过程。通过改变激光光场的时频域、强度和偏振以及横/纵向磁场强度来调控不同尺寸、不同组分或表面状态量子点的电子与原子核自旋态，探索相关的自旋物理及潜在应用。本项目不仅具有重要的基础研究意义，也在常温固态量子信息领域有重要的应用背景。

电子自旋有望用于实现量子信息处理。目前已被广泛研究的MBE生长的III-V 族低维材料其自旋调控实验通常只能在液氦低温条件下进行。实际器件化的需要希望有个室温的工作环境。本项目在室温条件下深入研究了各种II-VI族胶体量子点电子自旋弛豫动力学、操控及其应用拓展。研究发现，电子空穴电荷空间分离是实现室温下电子自旋长寿命和强自旋信号的关键因素。在自旋定向-探测装置的基础上，增加一载流子预泵浦光束，通过改变预泵浦-自旋定向光束之间的延迟时间以及激光重复频率和波长、预泵浦光的强度、材料表面状态，可以达到控制电子空穴电荷分离状态，进而实现自旋的高效调控。在室温下，从电子-空穴空间重合到空间分离，电子自旋寿命可从亚皮秒量级延长到纳秒量级。调节合适的预泵浦-自旋定向脉冲延迟时间以及激光强度，可使自旋信号增强一个量级。反过来，我们利用自旋信号的演化实现了由于载流子表面俘获所导致的电子空穴电荷分离超快动力学的探测。结果表明，CdS胶体量子点存在10 ps和100 ps量级的空穴俘获过程，量子点内核开始以带负电荷为主逐渐演化为以带正电荷为主，室温下正荷电状态的寿命长达数百微秒。本项目不仅具有重要的基础研究意义，也在常温固态量子信息领域有重要的应用背景。