Carrier-Induced and Coherent-Phonon-Controlled Band Renormalization in Quasi-One-Dimensional α-Bi4Br4

Directional control of band structures at ultrafast time scales is a key requirement for nanoscale optoelectronics. Here, we realize cooperative band engineering in the quasi-one-dimensional (quasi-1D) van der Waals topological insulator α-Bi4Br4 through joint carrier-phonon band renormalization. Femtosecond excitation drives quasi-1D carrier accumulation, generating a strongly nonlinear many-body band renormalization that redshifts an optical transition by up to ∼25 meV, with >90% polarization anisotropy. Superimposed Ag coherent phonons further modulate the same transition by ∼1 meV on intrinsic lattice time scales. By implementing phase-controlled dual-pump excitation, we achieve deterministic and reversible positioning of the renormalized band edge with ∼100 μeV precision within a ∼1.5 meV tuning window. First-principles calculations with charge-neutral photoexcitation constraints qualitatively reproduce the carrier-induced narrowing of the relevant interband transition, while phonon-mode analysis reveals that coherent Ag phonons modulate the same transition through interchain hopping and orbital polarization. These results establish cooperative carrier-phonon band renormalization as a scalable, all-optical mechanism for ultrafast, polarization-sensitive band engineering in quasi-1D van der Waals nanomaterials, enabling dynamically reconfigurable optoelectronic functionality.