Static-Transient Platform Reveals Multi-Band Transition Effects on Ultrafast Saturable Absorber

For decades, the screening strategies for nonlinear optical materials have adhered to a core principle: the direct bandgap of the saturable absorption material must be smaller than the operational photon energy. With the groundbreaking advancements in large-bandgap quantum materials research, such as transition metal dichalcogenides, the super-bandgap carrier excitation via 2-photon absorption effects is also observed, which reveals fundamental scientific principles of enhanced nonlinear optical responses. Although bandgap engineering can be a novel dimension in nonlinear optical device design, a huge theory–application gap persists in understanding how band engineering regulates material nonlinearity, severely limiting performance breakthroughs in ultrafast optical modulators. To explore the impact of bandgap engineering on these devices, we employed a bismuth-based topological insulator (Bi2Se3/ Bi2Te3/BiSbTeSe2) as a saturable absorber and pioneered the integration of conventional nonlinear optical characterization with time-stretch dispersive Fourier transform techniques. Across the nearto mid-infrared spectral range, we reveal the critical role of multi-band carrier transitions in governing nonlinear optical manifestations, particularly the regulation laws of modulation depth. These findings establish a multidimensional engineering framework for a saturable absorber design while inaugurating a new paradigm for ultrafast photonics research based on quantum state manipulation mechanisms.