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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