Bi₄Br₄-based saturable absorber with robustness at high power for ultrafast photonic device

Saturable absorption may be induced at critical pumping power by nonlinear optical effects of nanomaterials, thereby making it possible to generate a high-power ultrafast laser. Recently, Bi₄Br₄ is theoretically predicted to be a member of topological insulators and is expected to be a promising candidate for saturable absorbers (SAs). We show here that Bi₄Br₄ features a large modulation depth of 42.3%. The Bi₄Br₄-based SA enables mode-locking operation at the near-infrared range, as demonstrated here by a 1.5 μm fiber laser with a signal-to-noise ratio (SNR) of 90 dB and a pulse duration of 172 fs. Moreover, the robustness of the Bi₄Br₄-based SA at relatively high power is of particular interest, which can be proved by a laser's stable operation state. The strong optical nonlinearity and robustness provided by Bi₄Br₄ may arouse a growing upsurge in the innovation of high-power ultrafast photonic devices and further development of photon applications.