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

Germanium sulfide (GeS) represents a typical two-dimensional (2D) material that exhibits fascinating prospects for optoelectronic applications, and the understanding of its atomic-scale structure is crucial to tailoring its desired photonic properties. Here, we systematically investigate the atomic-scale structure of GeS and its nonlinear optical absorption properties. The GeS nanosheets of a thickness of 2-3 layers exhibit high optical nonlinearity with an effective nonlinear coefficient of -0.318 cm/GW, larger than that of black phosphorous by three orders, which can be applied for the generation of ultrashort-pulse lasers. We fabricate a Tm-doped passively mode-locked fiber laser based on a GeS saturable absorber and achieve a fundamental repetition rate of 9.7 MHz and a pulse duration as short as 1.56 ps at ∼2 μm. These findings suggest that 2D GeS nanosheets could be an efficient nonlinear material in the mid-infrared waveband with potential applications in integrated nonlinear photonics.