Controllable epitaxy of quasi-one-dimensional topological insulator α-Bi4Br4 for the application of saturable absorber

Bismuth bromide (alpha-Bi4Br4) can demonstrate various exotic topological states, including higher-order topological insulator with hinge states and quantum spin Hall insulator with helical edge states. To date, alpha-Bi4Br4 nanowires can be obtained by using the exfoliation method from the bulk. However, it is still a great challenge to efficiently prepare alpha-Bi4Br4 nanowires suitable for potential applications, e.g., saturable absorber in ultrafast pulsed fiber lasers. Here, we report the controllable growth of alpha-Bi4Br4 thin films consisting of nanowires via molecular beam epitaxy technique. We show that the morphology of the alpha-Bi4Br4 nanowires depends on the growth temperature and BiBr3 flux. In addition, we also achieve alpha-Bi4Br4 nanowires on NbSe2 and gold substrates. Furthermore, we performed the saturable absorption property of alpha-Bi4Br4 thin films with a modulation depth of 21.58% and mode-locking at 1556.4 nm with a pulse width of 375 fs in the pulsed fiber lasers. Those results demonstrate the synthesis of quasi-1D topological material alpha-Bi4Br4, which is expected to be used for the fundamental research of topological physics and potential applications in optical devices.

Automated summary

This study demonstrates the controllable growth of alpha-Bi4Br4 thin films consisting of nanowires using molecular beam epitaxy technique. The morphology of the nanowires is found to depend on the growth temperature and BiBr3 flux. The research also shows the saturable absorption property of the thin films and their potential application in pulsed fiber lasers.