# Saturable Absorption Dynamics of Highly Stacked 2D Materials for Ultrafast Pulsed Laser Production
## 3 Transition Metal Dichalcogenide SAs  
### Few-Layered MoTe\({}_{2}\)/WTe\({}_{2}\) SAs  
TMDCs with chemical formula MX\({}_{2}\) where M is a transition metal (Mo, W) and X is a group VI element (S, Se, Te) are one important family of 2D materials. The monolayer structure of 2D TMDCs is composed of two hexagonal planar arrangements of X atoms and the in-between plane of M atoms. The M atoms are covalently bonded with the X atoms in a trigonal prismatic arrangement [47; 48]. MoS\({}_{2}\) and WS\({}_{2}\) are representative TMDC materials and they have a 2H-hexagonal phase and a sizable electronic band gap, which can vary from 1.2 to 1.9 eV and 1.34 to 2.0 eV, respectively, depending on the layer stacking number. In contrast, monolayer MoTe\({}_{2}\) and WTe\({}_{2}\) have a smaller band gap less than 1 eV. Especially, naturally-formed WTe\({}_{2}\) has a distorted octahedral coordination about the tungsten atom, adopting a distorted orthorhombic phase (Td-WTe\({}_{2}\)) and the displacement of tungsten atoms with respect to those of 2H-WTe\({}_{2}\) increases the metallic bonding between tungsten atoms, creating the tungsten chains and rendering this material semimetal. [PERSON] et al. demonstrated that MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets could serve as high-power SAs for ultrafast-pulsed laser production [49] at the wavelength of 1500 nm. For the preparation of MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets, parent MoTe\({}_{2}\)/WTe\({}_{2}\) layered materials were exfoliated by an intercalation method using sodium deoxycholate bile salt (Figure 2). Compared to a micromechanical cleavage method, which is suitable for a high-quality micrometer-scale sample preparation [50], the liquid exfoliation method enabled mass production of mono- to few-layered MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets [51]. However, this method needs complicated processes such as mixing, sonification, centrifugation and supernatant filtering. In addition, the use of sodium deoxycholate bile salt surfactant increases the fabrication cost. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses indicated that few-layered MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets could be manufactured using the liquid exfoliation method (Figure 3). The MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets were subsequently mixed with polyvinyl alcohol (PVA) and deposited on a side-polished to measure their saturable absorption performance. Both MoTe\({}_{2}\)-PVA and WTe\({}_{2}\)-PVA SA modules showed a typical saturable absorption curve, and the modulation depth was measured as 0.9% and 1.5%, respectively. In this experiment, any nonlinear response was not observed from other fiber components, indicating that the saturable absorption solely originated from MoTe\({}_{2}\)/WTe\({}_{2}\) SA modules. Figure 1: Saturable absorption curve with parameters for characterization. By using erbium-doped fiber and MoTe\({}_{2}\) and/or WTe\({}_{2}\) SAs, pulsed laser system was configurated and its performance was investigated at the wavelength of 1500 nm. Efficient mode-locking was successfully attained using a MoTe\({}_{2}\) SA, producing the ultrafast pulsed lasers with a duration of 1.2 ps. Similar outstanding performance was also observed for the 1.2 ps. Figure 3: Characterization of MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets: (**a,b**) SEM image; (**c,d**) AFM image and (**e,f**) TEM image. The inset shows the corresponding suspension of the nanosheets. Reprinted with permission from [49]. Figure 2: (**a**) Schematic diagram of a liquid exfoliation method for preparing MoTe\({}_{2}\)/WTe\({}_{2}\) nanosheets. Raman spectra of (**b**) bulk MoTe\({}_{2}\) and (**c**) bulk WTe\({}_{2}\). Reprinted with permission from [49]. bare and/or PVA-mixed WTe\({}_{2}\) SAs in which mode-locking occurred at the input power of 35 and 19 mW, respectively. Compared to other TMDCs, MoTe\({}_{2}\) and WTe\({}_{2}\) have greater absorption coefficients and mode-locking was induced with a lower threshold power.