Polarization-tunable nonlinear absorption patterns from saturated absorption to reverse saturated absorption in anisotropic GeS flake and an application of all-optical switching

Due to the unique anisotropic chemical and physical properties, two-dimensional (2D) layered materials, such as IV-VI monochalcogenides with puckered honeycomb structure, have received considerable interest recently. Among the IV-VI layered MX (M = Ge, Sn; X = Se, S) compounds, germanium sulfide (GeS) stands out for its strongest anisotropic thermal conductivities and figure-of-merit values. Additionally, the layer-independent direct energy bands (E-g similar to 1.6 eV, E-1 similar to 2.1 eV) of GeS flake provide excellent insights into further applications as visible photodetectors. Herein, the polarization-tunable nonlinear absorption (NA) patterns of GeS flake have been systematically investigated. Specifically, both the polarization-dependent Raman spectroscopy and the linear absorption (LA) spectroscopy were employed to characterize the lattice orientation and absorption edges of the 251-nm GeS flake. Considering the low damage threshold of GeS flake, the GeS/graphene heterostructure was fabricated to increase the threshold without changing the nonlinear properties of GeS. Our NA results demonstrated that a 600-nm femtosecond laser with different polarizations would excite the saturated-absorption (SA) effect along armchair and reverse-saturated-absorption (RSA) effect along zigzag in the GeS/graphene heterostructure. Moreover, the function of the polarization-based GeS/graphene heterostructure all-optical switch was experimentally verified. Notably, thanks to the polarization-dependent NA patterns (SA/RSA) of GeS, the ON and OFF states of the all-optical switch can be accomplished by high and low transmittance states of continuous-wave laser (532 nm, 80 nW), whose state can be controlled by the polarization of femtosecond switching laser (600 nm, 35 fs, 500 Hz, 12 GW cm(-2)). The ON/OFF ratio can achieve up to 17% by changing polarization, compared with the ratios of 3.0% by increasing the incident power of switching light in our experiment. The polarization-tunable absorption patterns introduced in this work open up real perspectives for the next-generation optoelectronic devices based on GeS/graphene heterostructure.