Small but Efficient: Quasi‐1D Topological Insulator β‐Bi ₄ I ₄ Nanowires as a Saturable Absorber with High Modulation Depth and Broadband Operation

Abstract As a quasi‐1D topological insulator (TI) with a unique chain‐like atomic structure and anisotropic carrier transport characteristics, β‐Bi 4 I 4 has drawn extensive interest in quantum transport, pressure‐tunable superconducting phase transitions, and advanced electronic devices. However, the broadband photon absorption capacity of β‐Bi 4 I 4 , determined by its relatively small bulk bandgap, and especially its enormous potential in the field of ultrafast optical modulation systems, remains underexplored. This work develops β‐Bi 4 I 4 as a high‐performance saturable absorber, supporting mode‐locked operation of multi‐wavelength femtosecond fiber oscillators. The device exhibits distinct nonlinear saturable absorption: it shows >60% modulation depth across three common laser operation bands and promotes the formation of a mode‐locking state for Yb‐doped, Er‐doped, and Tm‐doped fiber oscillators. This study expands broadband photonics boundaries, provides a novel design for ideal mode‐locked modulators in ultrafast fiber oscillators, and promotes the integration of TI with nonlinear optical technologies.