1 Introduction 1.1 Overview of Dirac Semimetal 1.1.1 2D Dirac Semimetal 1.1.2 3D Dirac Semimetal 1.2 Intriguing Light-Matter Interaction in Dirac Semimetal 1.2.1 Ultrafast Dynamics and Population Inversion 1.2.2 Floquet Engineering in Dirac Semimetal References 2 Experimental Techniques 2.1 Angle-Resolved Photoemission Spectroscopy 2.1.1 Basic Principles 2.1.2 Instruments 2.2 Time-and Angle-Resolved Photoemission Spectroscopy 2.2.1 Basic Principles 2.2.2 Special Technical Considerations References 3 Development of Novel TrARPES with Tunable Probe Photon Energy for 3D Quantum Materials 3.1 Research Motivation 3.2 TrARPES with Tunable Probe Photon Energy 3.2.1 Experimental Setup 3.2.2 Tunable Probe Photon Energy 3.2.3 Considerations for the Time Resolution 3.2.4 TrARPES Functionality 3.3 Conclusions References 4 Chiral Symmetry Breaking in Kekul?-Ordered Graphene 4.1 Research Motivation 4.2 Experimental Evidence of Chiral Symmetry Breaking 4.2.1 Preparation of Kekul?-Ordered Graphene 4.2.2 Evidence of CSB 1: Band Gap Opening 4.2.3 Evidence of CSB 2: Kekul?-O Order 4.2.4 Evidence of CSB 3: Chirality Mixing 4.3 Conclusion References 5 Coexistence of Flat Band and Kekul? Order 5.1 Research Motivation 5.2 Extremely High Doping Level 5.3 Strong el-ph Coupling 5.4 Co-development of Flat Band and Kekul? Order 5.5 Conclusion References 6 Dirac Fermion Cooling in 3D Dirac Semimetal Cd3As2 6.1 Research Motivation 6.2 Revealing the 3D Dirac Cone 6.3 Photoexcited Carrier Dynamics 6.4 Ultrafast Thermodynamics 6.5 Population Inversion 6.6 Conclusion References 7 Summary and Prospects
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