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

• Few-layer NiTe 2 nanosheets are prepared by combining the convenient chemical synthesis and liquid phase exfoliation. • Two-dimensional semi-metallic NiTe 2 exhibits saturable absorption properties, which has been seldom reported. • Er-doped and Tm-doped mode-locked fiber lasers are realized using NiTe 2 -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 MoS 2 and WS 2 have narrow operating band and slow carrier recovery time, limiting their practical application. Among various kinds of TMDs, nickel ditelluride (NiTe 2 ) 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 NiTe 2 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 NiTe 2 nanosheets were prepared and SA device was further fabricated by depositing them on a tapered fiber. The saturable absorption property of NiTe 2 SA was confirmed by using a balanced twin-detector measurement system. With the use of NiTe 2 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 NiTe 2 SA. These results show that NiTe 2 is a promising SA material for ultrafast pulse laser.