Driving and detecting topological phase transition in noncentrosymmetric systems via an all-optical approach

The use of strong periodic light irradiation to drive Bloch state evolution and another weak light to probe this dynamic process is an effective yet underexplored strategy. Here, based on the first-principles calculations and Floquet theory, we systematically study the electronic structure evolution in a noncentrosymmetric system under a circularly polarized pump light. This process is analyzed using the Bernevig-Hughes-Zhang model, in which we elucidate that the pump light renormalizes the hopping integral and acts as an effective spin-orbit coupling, so that it reduces the system band gap. Using a monolayer BiSb as an exemplary material platform, we show that its band gap would close and reopen as the pump light intensity increases, corresponding to a topological phase transition (TPT) with distinct Chern numbers. We further propose that a weak probe light could capture such a TPT through second-order nonlinear optical responses, such as the bulk photovoltaic effect (BPVE) and second harmonic generation (SHG) process. Both the BPVE and SHG are largely enhanced in the Floquet-induced topologically nontrivial state. Importantly, the shift current flips direction under the TPT, offering an exquisite characteristic tool with high sensitivity to characterize the Chern number. Our work proposes an all-optical strategy to pump and probe the TPT in noncentrosymmetric systems, providing potential design strategy of dynamic nonlinear optical devices.