# Saturable Absorption Dynamics of Highly Stacked 2D Materials for Ultrafast Pulsed Laser Production
## 3 Transition Metal Dichalcogenide SAs
### Highly Stacked MoTe\({}_{2}\)/WTe\({}_{2}\) SAs  
Mono- and few-layer TMDCs have widely been investigated as SAs for ultrashort laser pulse generation, but their preparation is nontrivial and requires much expertise. In this regard, studies on the possible use of highly stacked TMDCs as SAs would not only be intriguing at the academic aspect, but also technically important for the development of cost-effective passively mode-locked laser technologies. Noticeably, through a combined experimental and theoretical approach, it was demonstrated that defective, highly stacked WTe\({}_{2}\) microflakes could be superb SAs for ultrafast mode-locked pulsed lasers, enabling the production of femtosecond lasers in a fiber cavity at the wavelength of 1550 nm [33]. To prepare WTe\({}_{2}\) microflakes suitable for this operation, commercial WTe\({}_{2}\) bulk crystal was roughly disrupted by ultrasonic treatment in distilled water for consecutive 3 h periods with 1 h in-between recess. Then, the solution of WTe\({}_{2}\) microflakes (3 mg mL\({}^{-1}\)) was taken from the top of the solution. Prior to fabrication of a SA fiber module, the solution was dropped on a glass substrate and dried for 24 h for structural characterization. The SEM and AFM images of the prepared WTe\({}_{2}\) microflakes showed that the lateral dimension of the WTe\({}_{2}\) microflakes was about 1.7 \(\upmu\)m and their thickness ranges from 80 to 150 nm (Figure 5). The interlayer spacing of WTe\({}_{2}\) crystals was known as 0.75 nm and thus they were 100-200 stacked layers of WTe\({}_{2}\). In-depth analysis showed that the WTe\({}_{2}\) microflakes had an average overall depth of 300 nm (from 100 to 1000 nm), being far from a mono- and/or few-layered 2D material regime. To fabricate a WTe\({}_{2}\) SA fiber module, the WTe\({}_{2}\) solution was mixed with PVA and dropped on the surface of a side-polished fiber. The dimension of the WTe\({}_{2}\)-PVA film was about 10 mm \(\times\) 9 mm and its thickness was measured as 2.3 \(\upmu\)m using a surface profiler (DetakXP Stylus Profiler, Bruker). The optical transmission curve of the WTe\({}_{2}\)-PVA deposited SA module was obtained using the input of a 1-ps mode-locked pulsed lasers with a wavelength of 1550 nm and a repetition rate of 14.15 MHz (Figure 6). This transmission curve was fitted to following formula for quantitative SA characterization [52]. Figure 4: Results of the WTe\({}_{2}\) mode-locked fiber lasers. (**a**) Spectrum, (**b**) autocorrelation trace, (**c**) pulse train and (**d**) RF spectrum. Reprinted with permission from [49]. \[T(I)=1-\Delta T\exp\biggl{(}\frac{-I}{I_{\text{sat}}}\biggr{)}-T_{\text{ns}} \tag{2}\]  
where \(T(I)\) is the transmission, \(\Delta T\) is the modulation depth, \(I\) is the input pulse energy, \(I_{\text{sat}}\) is the saturation energy and \(T_{\text{ns}}\) is the non-saturable loss. The modulation depth and saturation power were measured as 2.85% and 64.6W, respectively. The minimum insertion loss and polarization-dependent loss of the WTe\({}_{2}\)-PVA deposited SA module were 3.8 and 4 dB, respectively. WTe\({}_{2}\) is semimetal, but it is prone to oxidation and likely to induce defects in practical applications. To gain insights to this phenomenon, density functional theory (DFT) calculations were performed for the electronic band structures of a defect-free bulk WTe\({}_{2}\) and six monolayer WTe\({}_{2}\) systems with different oxidation states, which had chemical compositions (Figure 7) corresponding to the experimental analysis, and the monolayer structures of WTe\({}_{2}\) roughly stood for surface layer elements of WTe\({}_{2}\) microflakes. Figure 5: (**a**) Scanning electron microscope image and (**b**) atomic force microscopy image of the prepared WTe\({}_{2}\) microflakes on a glass substrate. (**c**) The height profile measured along the red line in (**b**). Reprinted with permission from [33]. Figure 6: (**a**) A schematic and a photograph of the prepared side-polished fiber deposited with a WTe\({}_{2}\)-PVA composite film.