Novel two-dimensional semi-metallic NiTe2 based saturable absorber for ultrafast mode-locked fiber laser

•Few-layer NiTe2 nanosheets are prepared by combining the convenient chemical synthesis and liquid phase exfoliation.•Two-dimensional semi-metallic NiTe2 exhibits saturable absorption properties, which has been seldom reported.•Er-doped and Tm-doped mode-locked fiber lasers are realized using NiTe2-based tapered fiber saturable absorber, respectively. Two-dimensional transition metal dichalcogenides (TMDs) have been widely used in ultrafast optics. However, the traditional TMDs such as MoS2 and WS2 have narrow operating band and slow carrier recovery time, limiting their practical application. Among various kinds of TMDs, nickel ditelluride (NiTe2) is a special type-II Dirac semi-metallic TMD material developed recently, showing great potential in ultrafast pulse generation due to its broadband optical response, ultrafast carrier recovery time, strong light-matter interaction and high chemical stability. Nevertheless, there has been no research reported on the NiTe2 based saturable absorber (SA) and its application in the mode-locked laser at the 1.5 and 2 μm regions. In this work, few-layer NiTe2 nanosheets were prepared and SA device was further fabricated by depositing them on a tapered fiber. The saturable absorption property of NiTe2 SA was confirmed by using a balanced twin-detector measurement system. With the use of NiTe2 SA, the Er-doped mode-locked fiber laser with a central wavelength of 1567.3 nm and a pulse width of 1.07 ps was achieved. Moreover, the Tm-doped mode-locked fiber laser with a central wavelength of 1966.08 nm was also obtained based on the NiTe2 SA. These results show that NiTe2 is a promising SA material for ultrafast pulse laser.