Spectroscopic Evidence of Ultrafast Topological Phase Transition by Light-Driven Strain.

Enabling reversible control over the topological invariants, transitioning them from nontrivial to trivial states, has fundamental implications for quantum information processing and spintronics. It offers a promising avenue for establishing an efficient on/off switch mechanism for robust and dissipationless spin-currents. While mechanical strain has traditionally been advantageous for such manipulation of topological invariants, it often comes with the drawback of in-plane fractures, rendering it unsuitable for high-speed, time-dependent operations. This study employs ultrafast optical and THz spectroscopy to explore topological phase transitions induced by light-driven strain in Bi₂Se₃. Bi₂Se₃ requires substantial strain for Z₂ switching. Our observations provide experimental evidence of ultrafast switching behavior, demonstrating a transition from a topological insulator with spin-momentum-locked surfaces to hybridized states and normal insulating phases under ambient conditions. Notably, applying light-induced strong out-of-plane strain effectively suppresses surface-bulk coupling, facilitating the differentiation of surface and bulk conductance even at room temperature─significantly surpassing the Debye temperature. We expect various time-dependent sequences of transient hybridization and manipulation of topological invariant through photoexcitation intensity adjustments. The sudden surface and bulk transport alterations near the transition point enable coherent conductance modulation at hypersound frequencies. Our findings on the potential of light-triggered ultrafast switching of topological invariants hold promise for high-speed topological switching and its related applications.