Ultrashort laser pulses can be used to manipulate quantum materials and probe the ultrafast dynamics, where the pump photon energy can strongly affect the light-matter coupling mechanisms. Here, by pumping hole-doped ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ using a low photon energy covering 135–330 meV, we reveal different excitations and relaxation dynamics upon above-gap and below-gap pumping by time- and angle-resolved photoemission spectroscopy. The above-gap pumping mainly leads to the injection of photoexcited carriers into the bulk conduction band and unoccupied surface states, while below-gap pumping leads to multiple sets of light-field dressed sidebands. In addition, the excited surface states above the Fermi energy also exhibit distinctive relaxation dynamics due to different relaxation channels involving intervalley, interband, and intraband scatterings. Our work reveals the essential role of pump photon energy on the light-matter coupling mechanisms, evolving from above-gap carrier injection to below-gap light-field dressing with distinct relaxation dynamics.

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• Electronic structure
• Light-matter interaction
• Dirac semimetal
• Topological insulators
• Time & angle resolved photoemission spectroscopy