狄拉克半金属因其独特的电子能带结构、新奇的物理特性尤其是丰富的光-物质相互作用，而受到广泛关注并且具有广阔的应用前景。因此，调控其电子能带结构以及研究其在光激发下的超快动力学过程具有重要的科学意义。本文研制了国际上首台具有可调谐探测光子能量的超快时间分辨角分辨光电子能谱系统，并利用该系统研究了两个典型的狄拉克半金属材料的电子结构调控和超快动力学过程，取得了以下创新性研究成果：（1）研制了国际上首套具有连续可调探测光子能量的超快时间分辨角分辨光电子能谱系统，其探测光子能量覆盖5.3 - 7.0 eV 范围，并且兼具有高能量、动量和时间分辨率（280 - 320 fs）。该系统的成功研制为三维量子材料的超快动力学和光诱导的电子结构调控提供了重要的机遇。（2）在石墨烯中通过锂插层实现了手征对称性破缺这一重要物理效应，并提供了手征对称性破缺的实验证据：狄拉克点处能隙的打开、实空间的Kekulé 序调制和相反手性的混合。此外，在该石墨烯体系中还观测到了强电声耦合作用以及与Kekulé 序强烈关联的1/4 填充的平带电子结构。该工作为研究手征对称性破缺相关物理及探索平带电子结构诱发的新奇物态提供了新的机遇。（3）首次直接揭示了三维狄拉克半金属Cd3As2 中光激发三维狄拉克费米子的超快动力学过程。通过电子弛豫速率随能量的线性依赖关系，揭示了带间弛豫的冷却过程，并且发现了具有长寿命的布居数反转效应，为探索奇异的瞬态激子绝缘体奠定了基础。本文从两个方面推进了狄拉克半金属电子结构这一研究领域。首先，该工作发现了Kekulé 序石墨烯这样一个新的石墨烯体系，为探索手征对称破缺相关物理包括电子分数化和拓扑效应以及平带诱导的新奇物态提供了新的机遇。其次，本论文成功解决了三维狄拉克半金属非平衡态电子结构研究中长久以来存在的实验挑战，使得直接在动量和能量空间中探索三维狄拉克半金属中的光-物质相互作用第一次成为了可能，特别是为研究三维拓扑材料中的光诱导拓扑相变迈出重要一步。
Dirac semimetals exhibit unique electronic band structure, novel physical properties with rich light-matter interaction, and potential applications. Revealing the band engineering and ultrafast dynamics of Dirac semimetals is therefore important. In this thesis, we develop the first ultrafast time- and angle-resolved photoemission spectroscopy (TrARPES) with tunable probe photon energy, and investigate the band structure engineering and ultrafast dynamics of Dirac semimetals by utilizing such a unique TrARPES system. Below are the major scientific achievements:(1) We report the development of a TrARPES system with a highly tunable probe photon energy from 5.3 to 7.0 eV. Our TrARPES system has high energy, momentum and time resolution (280–320 fs), which opens up new opportunities for exploring ultrafast dynamics in 3D quantum materials.(2) We realize the long-sought-after chiral symmetry breaking (CSB) via Li intercalation of graphene and provide direct experimental evidences. The CSB is evidenced by the gap opening at the Dirac point, Kekulé-O type modulation, and chirality mixing near the gap edge. In addition, we also report the experimental observation of an extended flat band below E? in this system with strong electron-phonon interaction which coexists and codevelops with the Kekulé order, thus providing opportunities for investigating CSB related physics, flat band related instabilities and its interplay with the Kekulé order.(3) We reveal the ultrafast dynamics of Dirac fermions in a model 3D Dirac semimetal Cd3As2 by using our TrARPES system with a tunable probe photon energy. The energy- and momentum-resolved relaxation rate shows a linear dependence on the energy, suggesting Dirac fermion cooling through intraband relaxation. Moreover, a population inversion with a long lifetime of 3.0 ps is reported, which paves an important step toward exploring the possible dynamic exotic excitonic insulator.This thesis advances the research of the electronic structure of Dirac semimetal from two aspects. First, it identifies the Kekulé ordered graphene as a new system for exploring chiral symmetry breaking related physics including electronic fractionalization and topology effect, and flat band induced instability, including chiral superconductivity, especially providing a rare system to investigating their interplay. Secondly, this thesis solves the long-standing challenge of directly visualizing the non-equilibrium band structure of 3D Dirac semimetal and opens up new opportunities for exploring the light-matter interaction in 3D quantum materials, especially the light-induced topological phase transitions in 3D topological materials through Floquet engineering.
