Ⅱ型半导体纳米异质结的超快光物理研究

Semiconductor heterojunctions with type-II band alignment are essential units for demonstrating optoelectronic devices. Due to the effect of quantum confinement, the physical properties in nano-architectured heterojunctions are significantly modified with the formation of charge-transfer excitons in an ultrafast scale post-excitation, which is vital for the implications in semiconductor devices. In this project, we aim to uncover the physical mechanisms underlying the charge-transfer excitons and explore the pivotal effect of quantum coherence involving in the process of charge separation. We plan to carry out experiments on two model systems，i.e., type-II semiconductor nanocrystals and Type-II heterojunctions of 2D semiconductors, with the techniques of transient absorption and two-dimensional quantum coherence spectroscopy. The experimental data will be acquired to construct the ultrafast dynamics of charge separation, to investigate the exciton coherence and their entanglement during charge separation, and to characterize the effect of excitonic interaction on the dynamics of charge separation. Our research will provide an explanation of extremely ultrafast charge separation from the view of quantum coherence, which is of great value for demonstrating new schemes to pursue highly efficient optoelectronic conversion.

Ⅱ型半导体异质结结构是光电转换器件的基本单元，当异质结尺寸进入纳米尺度时，量子限制效应重新塑造异质结的物理特性，在极快时间尺度形成传统异质结中本不存在的电荷转移激子，这一过程对于Ⅱ型半导体纳米异质结的应用具有重要意义。本项目尝试揭示这一重要过程的物理机制，探索量子相干特性在电荷转移激子形成过程中的关键作用。为此，我们将以Ⅱ型半导体量子点和二维异质结结构为样品模型，结合<20飞秒的瞬态吸收光谱和二维量子相干光谱学实验技术，建立Ⅱ型纳米半导体异质结的激子分离的极超快动力学过程，确立各激子态之间的量子相干耦合以及相干性与电荷分离之间的关系，揭示激子相互作用对激子分离动力学和激子间相干耦合的影响。这一研究将利于揭示Ⅱ型能级排列的半导体纳米界面的电荷转移过程中的量子相干效应，探索实现高效光电转换的半导体光电器件的新机制。

本项目针对二型半导体半导体异质结的超快光物理过程，开发高时间分辨的瞬态光谱学方法，探究纳米材料以及异质结电荷转移过程中可能存在的非经典效应。项目组成果结合非共线参量放大和主动锁相技术，开发时间分辨<10 fs的瞬态吸收和二维量子相干光谱学技术，相位稳定性好于波长/300，主要技术指标达到国际领先水平，获得专利授权；系统的开展了钙钛矿量子点的超快动力学研究，观测到暗激子态存在的光谱学证据，确认了双激子态光学增益机制，实现了低阈值双光子激发激光器，论文发表于JACS，入选高被引论文；深入研究了金属半导体界面热载流子转移的动力学过程，借助钙钛矿材料的高激发态密度，实现了高效的相干热载流子转换；系统研究了单线态裂分机制，有机半导体体异质结的电荷分离机制，提出极化子主导的电荷分离新机制。

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