Nonlinear absorption property and carrier dynamics in MoS₂/Graphene van der Waals heterostructures

Recently, molybdenum disulfide (MoS₂) nanosheets have been used in passively mode-locked laser due to excellent saturable absorption (SA) property. To enhance their optical nonlinearity, some heterostructures such as MoS₂/Graphene (MoS₂/G) have been designed, but the carrier dynamics in the materials have rarely been reported. Here, the nonlinear response of MoS₂ and MoS₂/G heterostructures nanosheets with different thickness are studied via nanosecond laser Z-scan and femtosecond time-resolved transient absorption (TA) technique. The results reveal that the ground state bleaching take place under intense laser excitation, which would block the further absorption of incident light, causing a strong SA effect of the materials. With increasing the thickness of MoS₂, electron diffusion take place between adjacent layers, resulting in a slow recombination of excited carriers and the enhancement of SA effect in MoS₂. In MoS₂/G, excited electrons could transfer from MoS₂ to graphene and the interlayer carrier diffusion in MoS₂ could be enhanced, prolonging the decay time of ground state bleaching effect. The slower carrier relaxation time in MoS₂ would lead a stronger SA effect for nanosecond laser pulse. This work could provide an effective strategy for designing saturable absorbers in mode-locked lasers using MoS₂ heterostructures.