Atomic-scale structure and nonlinear optical absorption of two-dimensional GeS

•Resolving spatially and identifying chemical atoms in layered GeS material.•GeS nanosheets with a thickness of 2, 3 layers exhibit high optical nonlinearity.•Realizing a passively mode-locked fiber laser based on a GeS saturable absorber.•2D GeS is a promising nonlinear material for mid-infrared waveband applications.