Ultrafast Saturable Absorption of Two-Dimensional MoS2 Nanosheets

Employing high-yield production of layered materials by liquid-phase exfoliation, molybdenum disulfide (MoS2) dispersions with large populations of single and few layers were prepared. Electron microscopy verified the high quality of the two-dimensional MoS2 nanostructures. Atomic force microscopy analysis revealed that similar to 39% of the MoS2 flakes had thicknesses of less than 5 nm. Linewidth and frequency difference of the E-2g(1) and A(1g) Raman modes confirmed the effective reduction of flake thicknesses from the bulk MoS2 to the dispersions. Ultrafast nonlinear optical (NLO) properties were investigated using an open-aperture Z-scan technique. All experiments were performed using 100 fs pulses at 800 nm from a mode-locked Ti:sapphire laser. The MoS2 nanosheets exhibited significant saturable absorption (SA) for the femtosecond pulses, resulting in the third-order NLO susceptibility Im chi((3)) similar to 10(-15) esu, figure of merit similar to 10(-15) esu cm, and free-carrier absorption cross section similar to 10(-17) cm(2). Induced free carrier density and the relaxation time were estimated to be similar to 10(16) cm(-3) and similar to 30 fs, respectively. At the same excitation condition, the MoS2 dispersions show better SA response than the graphene dispersions.