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.

As a handicap for passive mode-locked lasers to operate stably for a long time, the undesirable damage threshold of saturable absorption devices means that it is difficult to maintain a normal operation state under the irradiation brought by ultrashort pulses. To reduce the damage probability when the saturable absorption devices interact with an ultrafast laser, some technologies have been developed to resist the high-energy irradiation of an ultrafast laser that will also increase the difficulty of laser construction at the same time. In this paper we put forward a new research idea to improve the low-damage threshold problem: optimizing the optical transmittance, which will avoid dissociation due to excessive heat accumulation. Using the melt and machine exfoliation method, the high optical transmittance (>95%) saturable absorption devices based on Bi 2 Se 3 are prepared, whose damage threshold (2.45 mJ/cm 2 ) is nearly 5 times higher than commercial SESAM. The devices applied to generate femtosecond pulses exhibit exciting nonlinear optical effects, which enable the mode-locked fiber laser to operate with a pulse duration of 195 fs and a signal-to-noise ratio of 81.73 dB. This discovery not only proves that the passive mode-locked laser-based high transmittance Bi 2 Se 3 can obtain reliable pulse output, but also provides a new perspective for the selection of saturable absorption materials in the future.

Topological insulator materials, including Bi2Te3, Sb2Te3, Sb2Te3 and Bi2Se3, have attracted some attention due to their narrow band gap, high carrier mobility, wide spectral absorption range and other characteristics. Compared with the traditional topological insulator composed of two elements, the new multi-compound topological insulator material can integrate the physical advantages of each element, which is helpful to build an experimental platform with rich physical properties. In this work, the nonlinear optical characteristics of the quaternary material BiSbTeSe2 is reported in the erbium-doped fiber laser. Using the BiSbTeSe2 as a saturable absorber material, the passive Q-switched and mode-locked fiber lasers are achieved. The pulse duration and SNR of the Q-switched fiber laser are 854 ns and 70 dB, respectively. Meanwhile, the pulse duration and SNR of the mode-locked fiber laser are 259 fs and 87.75 dB, respectively. This work proves that the BiSbTeSe2 has a considerable application prospect as a saturable absorber in fiber lasers, and provides a new reference for the selection of high-performance saturable absorber materials.

Fiber lasers with different net dispersion cavity values can produce some types of solitons, and rich nonlinear dynamics phenomena can be achieved by selecting different saturable absorbers. In this paper, a new layered high-entropy van der Waals material (HEX) (Mn, Fe, Co, Ni) PS3 was selected as a saturable absorber to achieve a high-power laser output of 34 mW. In addition, the wavelength can be dynamically tuned from 1560 nm to 1531 nm with significant dual-wavelength phenomena at 460 fs pulse duration.

Bismuth bromide (α-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, α-Bi4Br4 nanowires can be obtained by using the exfoliation method from the bulk. However, it is still a great challenge to efficiently prepare α-Bi4Br4 nanowires suitable for potential applications, e.g., saturable absorber in ultrafast pulsed fiber lasers. Here, we report the controllable growth of α-Bi4Br4 thin films consisting of nanowires via molecular beam epitaxy technique. We show that the morphology of the α-Bi4Br4 nanowires depends on the growth temperature and BiBr3 flux. In addition, we also achieve α-Bi4Br4 nanowires on NbSe2 and gold substrates. Furthermore, we performed the saturable absorption property of α-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 α-Bi4Br4, which is expected to be used for the fundamental research of topological physics and potential applications in optical devices.