Floquet engineering of topological surface states in the ultrathin limit

Floquet states are coherent quantum states that emerge in time-periodically driven systems, exhibiting replica bands separated by characteristic energy intervals. Previous studies have demonstrated that such field-dressed transient states can be realized in mid-infrared-pumped topological surface states of Bi₂Se₃ single crystals, with their band structures directly visualized by angle-resolved photoemission spectroscopy (ARPES). Moreover, circularly polarized light can break time-reversal symmetry and induce a light-driven Chern insulator state, providing a potential route to explore the quantum anomalous Hall effect in thin films. This motivates a deeper understanding of Floquet dynamics in the ultrathin limit, where interface effects such as band bending, together with the hybridization between top and bottom surface states, can play a crucial role. Here, using time-resolved ARPES, we systematically studied Floquet states in Bi₂Se₃ thin films of varying thickness. Unexpectedly, we observed a non-monotonic evolution of the hybridization gap between different Floquet sidebands as a function of film thickness. Unlike in single crystals, where the topological surface-state wave function is localized near the surface due to the bulk band gap, thin films exhibit strong coupling between quantum well states and topological surface states, leading to more intricate Floquet dynamics. Our findings reveal the critical interplay between light-matter interaction and quantum confinement in the ultrathin regime and provide insights for Floquet engineering in other quasi-2D systems.