Direct Observation of a Photoinduced Topological Phase Transition in Bi-Doped (Pb,Sn)Se.

Ultrafast photoexcitation offers a novel approach to manipulating quantum materials. One of the long-standing goals in this field is to achieve optical control over topological properties. However, the impact on their electronic structures, which host gapless surface states, has yet to be directly observed. Here, using time- and angle-resolved photoemission spectroscopy, we visualize the photoinduced evolution of the band structure in Bi_{y}(Pb_{1-x}Sn_{x})_{1-y}Se(111) films from topological to trivial insulators. Following near-infrared ultrafast laser excitation, we observe that the topological surface state opens a substantial gap of up to 0.1 eV. Considering the topological phase diagram associated with lattice distortion and atomic displacement, we show that a uniaxial strain generated by the ultrafast optical pulse is sufficiently effective and strong for the observed topological phase transition. Our Letter highlights the potential of optical tuning of materials through laser excitation to control topological properties on ultrafast timescales.