Polarization-resolved second-harmonic generation (SHG) is reported in highly doped ${\mathrm{Bi}}_1{\mathrm{Te}}_1$, dual topological insulator, demonstrating the role of Rashba spin-split states (RSS) in driving a magnetization-induced nonlinear optical response. Steady-state measurements reveal an enhancement of the SHG signal under circularly polarized probe excitation, evidencing a magnetization-induced susceptibility ${\chi }^{\left(2\right)}\left(M\right)$. Time-resolved SHG precisely disentangles the contributions of the electric-field-induced SHG and the magnetization-induced SHG. A linearly polarized probe measures the relaxation of the screened electric field in the space-charge region. In contrast, with a circularly polarized probe, the signal strongly depends on the pump polarization and even reverses sign, indicating the influence of magnetization. Comparing experimental results with theoretical calculations of second-order conductivity and the induced magnetization of Rashba states under circularly polarized excitation using Floquet formalism yields an estimated Rashba spin-orbit coupling parameter of the order of a few eV Å, evidencing the presence of strong spin-orbit coupling in the system. Moreover, the theoretical framework successfully captures the observed dynamics and fluence dependence, in close agreement with experiments. These results establish ${\mathrm{Bi}}_1{\mathrm{Te}}_1$ as a prototypical platform for exploring magnetization-driven nonlinear phenomena and demonstrate the utility of SHG as a sensitive probe of strong spin-orbit-coupled systems.

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• Topological insulators
• Bloch-Floquet theorem
• Green's function methods
• Optical second-harmonic generation
• Time-resolved reflection spectroscopy