1 Introduction  1
1.1   Overview of Dirac Semimetal  1
1.1.1   2D Dirac Semimetal  1
1.1.2   3D Dirac Semimetal  3
1.2   Intriguing Light-Matter Interaction in Dirac Semimetal  4
1.2.1   Ultrafast Dynamics and Population Inversion  4
1.2.2   Floquet Engineering in Dirac Semimetal  6
References  10

2 Experimental Techniques  15
2.1   Angle-Resolved Photoemission Spectroscopy   15
2.1.1   Basic Principles  15
2.1.2   Instruments  18
2.2   Time- and Angle-Resolved Photoemission Spectroscopy  20
2.2.1   Basic Principles  21
2.2.2   Special Technical Considerations 22
References  24

3 Development of Novel TrARPES with Tunable Probe Photon Energy for 3D Quantum Materials  27
3.1 Research Motivation  27
3.2 TrARPES with Tunable Probe Photon Energy 28
3.2.1 Experimental Setup 28
3.2.2 Tunable Probe Photon Energy 30
3.2.3 Considerations for the Time Resolution 33
3.2.4 TrARPES Functionality 39
3.3 Conclusions 40
References 40

4 Chiral Symmetry Breaking in Kekulé-Ordered Graphene 43
4.1 Research Motivation 43
4.2 Experimental Evidence of Chiral Symmetry Breaking 44
4.2.1 Preparation of Kekulé-Ordered Graphene 44
4.2.2 Evidence of CSB 1: Band Gap Opening 48
4.2.3 Evidence of CSB 2: Kekulé-O Order 50
4.2.4 Evidence of CSB 3: Chirality Mixing 51
4.3 Conclusion 55
References 55

5 Coexistence of Flat Band and Kekulé Order 59
5.1 Research Motivation 59
5.2 Extremely High Doping Level 60
5.3 Strong el-ph Coupling 62
5.4 Co-development of Flat Band and Kekulé Order 63
5.5 Conclusion 65
References 66

6 Dirac Fermion Cooling in 3D Dirac Semimetal Cd3As2 69
6.1 Research Motivation 69
6.2 Revealing the 3D Dirac Cone 70
6.3 Photoexcited Carrier Dynamics 71
6.4 Ultrafast Thermodynamics 73
6.5 Population Inversion 73
6.6 Conclusion 76
References 76

7 Summary and Prospects 79
References 80

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 .rst 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 major scienti.c 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 inter-calation of graphene and provide direct experimental evidences. The CSB is evidenced by the gap opening at the Dirac point, Kekeulé-O type modulation, and chirality mixing near the gap edge. In addition, we also report the experi-mental observation of an extended .at band below EF in this system with strong electron-phonon interaction which coexists and codevelops with the Kekulé order, thus providing opportunities for investigating CSB-related physics, .at 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 identi.es the Kekulé-ordered graphene as a new system for exploring chiral symmetry breaking-related physics including electronic frac-tionalization and topology effect and .at band-induced instability, including chiral 
Abstract 
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 opportu-nities for exploring the light-matter interaction in 3D quantum materials, especially the light-induced topological phase transitions in 3D topological material through Floquet engineering. 
Keywords Dirac semimetal · Band engineering · Ultrafast dynamics · Angle-resolved photoemission spectroscopy (ARPES) · Time-and angle-resolved photoe-mission spectroscopy (TrARPES)