Nonlinear absorption properties 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. (c) 2020 Elsevier Ltd. All rights reserved.