| 题名 | 二维半导体微区非线性吸收特性研究 |
| 作者 | 李源鑫 |
| 文献子类 | 博士 |
| 导师 | 王俊 |
| 关键词 | 二维半导体 2D semicondutors 非线性光学 nonlinear optics 饱和吸收 saturable absorption 双光子吸收 two-photon absorption 二硫化钼 MoS2 |
| 其他题名 | Nonlinear Optical Absorption Investigation in Microdomain of Two-dimensional Semiconductors |
| 英文摘要 | 独特的量子限制效应和层间微扰的缺失使二维半导体在基础理论、光/电子学等领域有重要研究价值。近年来，利用少（多）层过渡金属硫系化合物及其复合材料作为可饱和吸收体应用于固体或光纤激光器中以产生调Q或锁模激光脉冲的研究发展迅速，但由于所采用样品的层数和结构的不均一，其物理机理以及精准层数/结构的二维半导体的非线性吸收机制仍不清晰。另一方面，单层二维半导体具有和其体材料以及少/多层显著不同的物理特性，然而关于其非线性非参量过程的研究的匮乏大大限制了其在光子学领域的应用潜力。因此，对二维半导体开展系统的非线性吸收性质的研究是拓展其在光子学领域应用的重要基础研究方向。基于此，本论文对精确层数和结构的二维半导体在近红外波段的非线性吸收特性展开了理论和实验研究，主要研究内容包括： 第一章绪论首先综述了二维材料的研究进展，详细介绍了以过渡金属硫系化合物为代表的二维半导体的物理特性和应用研究，然后从其非线性参量和非参量过程两个方面阐述了二维半导体光子学特性和应用的研究现状以及存在的问题，指出关于其非线性吸收物理机制的研究是二维半导体应用于光子学领域的重要方向。 第二章主要论述了理论基础、实验手段和研究方法。从能带结构和激子效应出发，结合非线性传输方程，给出了二维半导体中双光子吸收、饱和吸收的物理模型，揭示其中的物理本质；从传统的非线性测量手段——强度扫描出发，结合二维半导体形貌结构的特点，构建了适用于微区样品非线性吸收特性研究的显微非线性强度扫描系统，并利用LabVIEW和MATLAB分别开发了图形用户界面控制程序和相应的理论拟合程序；介绍了利用化学气相沉积原理制备过渡金属硫系化合物的方法及其表征手段。 第三章主要研究了化学气相沉积制备二硫化钼的生长机理、光物理特性和光学对比度。从二硫化钼晶畴的光学显微镜、扫描电子显微镜、透射电子显微镜和原子力显微镜图片分析了二维半导体的生长机理、晶格结构和表面形貌特征等；从拉曼光谱、光致发光光谱研究了二硫化钼的层数依赖关系、晶格振动特性、激子跃迁特性；首次提出利用特征矩阵计算二维材料光学对比度的方法，理论模拟了二硫化钼/石墨烯异质结的光学对比度，计算了不同层数的二硫化钼在石英基底上的光学对比度，通过和实验值对比证实了该方法的有效性和可靠性，比传统的菲涅尔理论更加精确和简便。 第四章主要研究了单层和多层二硫化钼在1030 nm、340 fs激光脉冲作用下的非线性吸收特性。利用显微非线性强度扫描系统研究了二硫化钼单晶和金字塔结构的非线性传输特性，揭示了单层二硫化钼的巨双光子吸收及其饱和效应，推导出其双光子吸收系数高达103量级，饱和光强为~ 64.5 GW/cm2，指出了其暗态激子共振吸收的物理本质，实验上观察到的双光子荧光是其双光子吸收效应的重要依据；揭示了多层二硫化钼是一种慢饱和吸收体，其基态吸收截面为8.7×10-17 cm2，激发态吸收截面和基态吸收截面之比为0.18，指出边缘态和缺陷态是引起其单光子吸收饱和的主要原因，研究结果是多层二硫化钼在近红外波段可以作为饱和吸收体的直接证据。 第五章系统研究了二硫化钼、二硫化钨、二硒化钼和二硒化钨四种不同厚度的二维半导体在1030 nm波段的双光子吸收饱和效应。从实验上观察到了二维半导体不同于传统非线性吸收材料包括半导体和有机分子等的独特的双光子吸收饱和效应，利用四种双光子吸收模型系统研究了其物理机制，揭示了四种二维半导体在双光子吸收过程中属于均匀加宽体系；发现双光子吸收系数和饱和光强具有明显的层数依赖特性；通过三能级的激子动力学研究，揭示了暗态激子吸收及其快速弛豫、基态电子消耗和第一激发态的长寿命是导致双光子吸收饱和的主要因素。研究了二维半导体对飞秒高斯脉冲时间特性和空间特性的调制，指出其应用于光学脉冲调制领域的潜在价值。; The unique quantum confinement effect and the lack of interlayer perturbations make the two-dimensinal semiconductors intriguing materials for research in fundmental/theortical physics and electronics/photonics applications. Recently, it attracts a lot of interest that few-layer transition metal dichalcogenides (TMDCs) and their composites can be used as saturable absorbers in solid or fiber lasers to generate Q-switched or mode-locked laser pulses, however, the nonlinear optical (NLO) mechanism of which is still in dispute and unclear due to the ununiformity of the few-layer TMDCs in thickness and construction. On the other hand, the monolayer of TMDCs has completely distinct physical properties comaring with the bulk or few-layer counterpart, but the lack of the study on their nonlinear nonparametric process limits their application potential in photonic field. Therefore, it is vital to carry out the systematic study on the NLO properties of 2D semicondutors, which is also a fundamental research subject for their potential photonic applications. In this thesis, we investigate the nonlinear absorption property of TMDCs with precise layer-number and domain structures both in experiments and theory systematically, and the main contents include: In the introduction chapter, we review the research progress of two-dimentional materials, and introduce the studies on the physical properties and application of two-dimensional semiconductors represented by TMDCs in detail. Then we review the research status and exsiting problems of the nonlinear parameteric and nonparameteric process study in 2D semiconductors, and point out that the nonlinear absorption research are very important for understanding these materials and applying them to photonics field. In chapter 2, we discuss the theoretical basis, experimental means and research methods. The two-photon absorption (TPA) and slow/fast saturable absorption models are deduced based on the energy band structure and excitonic effect of 2D semiconductor combined with the NLO propagation equation, and the physical nature is also revealed. We develop the microscopic nonlinear intensity scanning (μ-I-scan) system based on the traditional intensity scan (I-scan) method to measure the nonlinear transmission of microdomain TMDCs, and develop the graphical user interface (GUI) software using LabVIEW to control this system, and also write the theoretical fitting software using Matlab. At last, we introduce the preparation methodology and the characterization methods of TMDCs. The third chapter mainly studies the growth mechanism, the photophysical properties and optical contrast of MoS2 prepared using the chemical vapor deposition (CVD) method. We analyze the growth mechanism, lattice structure and surface morphology of 2D semiconductors based on the optical microscopy, scanning electron microscopy, transmission electron microscopy and atomic force microscopy images. We study the layer-number dependent properties, the lattice vibration and excitonic transition properties of MoS2 using Raman and photoluminescence spectrum. For the first time we propose the method to calculate the optical contrast of layered materials on arbitrary substrate using the characteristic matrix, and simulate the optical contrast of graphene/MoS2 heterostructure. In addition, we calculate the optical contrast of MoS2 with different layers on quartz, and demonstrate this method is reliable and effective by comparing the theoretical values with the experimental ones, and find it is more precise and convinent than Fresnel theory. In chapter 4, the the nonlinear absorption characteristics of monolayer and multilayer molybdenum disulfide under 1030 nm and 340 fs laser pulse are studied in depth. The nonlinear transmission of molybdenum disulfide monocrystalline and pyramid structures are studied using the μ-I-scan system. The giant TPA and its saturation effect of single layer molybdenum disulfide is revealed, and the TPA coefficient was deduced to be as large as 103 cm/GW, and the saturation intensity is ~ 64.5 GW/cm2, which is attributed to the resonant absorption of the dark state excitons. The two-photon excited frequency up-conversion fluorescence observed in the experiment is an important evidence for its TPA effect. We also demonstrate that MoS2 is a slow saturable absorber with a ground state absorption cross section of 8.7 × 10-17 cm2 and a ratio of excited state absorption cross section to ground state absorption cross section of 0.18, indicating that the edge states and the defect states are the main causes of its single photon absorption saturation. The results of this study are direct evidence that few-layer molybdenum disulfide can be used as a saturable absorber in the near infrared band. In the fifth chapter, the TPA saturation effect of four kinds of two-dimensional semiconductors of MoS2, WS2, MoSe2 and WSe2 at 1030 nm is systematically studied. The unique TPA saturation phenomena are observed directly in the NLO measuremts, which are distinct from the tranditional semiconductors and organic molecules. Four TPA models are used to reveal the physical mechanism of these TMDCs, and the results indicate that all of the nanofilms belong to homogeneously broadened media. The TPA coefficient and the saturable intensity have obvious layer-dependent properties. The three-level exciton kinetic study reveals that the dark exciton absorption and its fast relaxation, ground state electron depletion and long life time of the first excited state are the main factors leading to TPA saturation. The modulation of the temporal and spatial characteristics of the femtosecond Gaussian pulse is studied, and the result indicate that 2D semiconductors have potential value in photonics application. |
| 学科主题 | 材料学 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/30930]  |
| 专题 | 中国科学院上海光学精密机械研究所 |
| 作者单位 | 中国科学院上海光学精密机械研究所 |
| 推荐引用方式 GB/T 7714 | 李源鑫. 二维半导体微区非线性吸收特性研究[D]. |
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