Abstract 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)

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