Robust Edge States of Quasi-1D Material Ta<sub>2</sub>NiSe<sub>7</sub> and Applications in Saturable Absorbers

The\nhelical edge states (ESs) protected by underlying <i>Z</i><sub>2</sub> topology in two-dimensional topological insulators (TIs)\narouse upsurges in saturable absorptions thanks to the strong photon–electron\ncoupling in ESs. However, limited TIs demonstrate clear signatures\nof topological ESs at liquid nitrogen temperatures, hindering the\napplications of such exotic quantum states. Here, we demonstrate the\nexistence of one-dimensional (1D) ESs at the step edge of the quasi-1D\nmaterial Ta<sub>2</sub>NiSe<sub>7</sub> at 78 K by scanning tunneling\nmicroscopy. Such ESs are rather robust against the irregularity of\nthe edges, suggesting a possible topological origin. The exfoliated\nTa<sub>2</sub>NiSe<sub>7</sub> flakes were used as saturable absorbers\n(SAs) in an Er-doped fiber laser, hosting a mode-locked pulse with\na modulation depth of up to 52.6% and a short pulse duration of 225\nfs, far outstripping existing TI-based SAs. This work demonstrates\nthe existence of robust 1D ESs and the superior SA performance of\nTa<sub>2</sub>NiSe<sub>7</sub>.