van der Waals Layered Tin Selenide as Highly Nonlinear Ultrafast Saturable Absorber

Saturable absorbers (SAs) are materials that can generate ultrashort pulse lasers via mode locking due to optical absorption nonlinearity. Recently, layered 2D materials have gained increasing attention as next-generation SAs. Layered transition-metal monochalcogenides (TMMCs) can provide the source for a large family of 2D materials with broad chemical composition while exhibiting superior structural features for SA, but as far as it is known, none of these is studied for SA applications to date. Here, for the first time, the nonlinear optical absorption properties of SnSe which belongs to accordion-like layered TMMCs are investigated along with in-depth exploration of its laser mode-locking performance. Notably, SnSe exhibits a highly nonlinear modulation depth of 7.1% and readily generates femtosecond pulse lasers with a duration of 610 fs at 1560 nm, whose combined performance surpasses that of any 2D-material-based SA established so far. Density functional theory calculations show that the bandgap of SnSe can greatly vary depending on layer thickness, indicating its broadband applicability. This study clearly shows that layered TMMCs hold great potential for advanced SAs, significantly extending the horizon of mode-locking materials for innovative laser technologies.