# 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
(**b**) Measured nonlinear saturable absorption curve of the side-polished fiber deposited with a WTe\({}_{2}\)-PVA composite film. Reprinted with permission from [33]. The calculated electron band structures of these WTe\({}_{2}\) systems were presented in Figure 8. The result for defect-free bulk WTe\({}_{2}\) showed a good agreement with that of previous studies, validating the employed computation method [53; 54] Pristine monolayer WTe\({}_{2}\) was semimetal, the same as defect-free bulk WTe\({}_{2}\), and no energy band was observed in tungsten-deficient WTe\({}_{2}\) (W:Te = 1:2.13). In contrast, small band gaps were observed in tellurium-deficient cases (0.033 eV for W:Te = 1:1.875 and 1:1.625). Meanwhile, oxidized WTe\({}_{2}\) had rather large band gaps ranging from 0.1 to 0.2 eV. The electronic band gap should decrease upon layer stacking due to alleviated quantum confinement. [55]. Based on these facts, it was supposed that WTe\({}_{2}\) could still operate at the near-infrared ranges (0.9-1.7 eV) even for severe surface oxidation. Finally, the WTe\({}_{2}\) SA module was incorporated into a ring-cavity erbium-doped fiber laser to investigate the mode-locking performance of a highly stacked WTe\({}_{2}\) SA (Figure 9). Stable mode-locked pulsed lasers were produced by increasing the pump power to 25 mW together with careful polarization adjustment (Figure 10a). The period and repetition rate of the generated pulses were 71.5 ns and 13.98 MHz, respectively, both of which agree well  
Figure 7: Atomic structures of (**a**) bulk WTe\({}_{2}\), (**b**) 1 L W\({}_{8}\)Te\({}_{18}\)O\({}_{5}\) (W:Te = 1:2.25), (**c**) 1 L W\({}_{8}\)Te\({}_{14}\)O\({}_{9}\) (W:Te = 1:1.75), (**d**) 1 L W\({}_{16}\)Te\({}_{32}\) (W:Te = 1:2), (**e**) 1 L W\({}_{15}\)Te\({}_{32}\) (W:Te = 1:2.13), (**f**) 1 L W\({}_{16}\)Te\({}_{30}\) (W:Te = 1:1.875) and (**g**) 1 L W\({}_{16}\)Te\({}_{26}\) (W:Te = 1:1.625). Tungsten, tellurium and oxygen are shown in dark two-tone gray, light-two-tone gray and dark one-tone gray, respectively. Reprinted with permission from [33]. with the round-trip time and fundamental resonance frequency of the 13.69-m fiber cavity system. The optical spectrum of the pulses is shown in Figure 10b and the sech\({}^{2}\) fitting for solitons indicated that they have the center wavelength of 1556.2 nm and 3 dB bandwidth of 4.14 nm. Using a two-photon absorption-based autocorrelator, the temporal width of the pulses was directly measured as 770 fs (Figure 10c), and the time-bandwidth product was 0.39, indicating that the output pulses were slightly chirped. In the electrical spectrum analysis, a sharp strong peak with a fundamental repetition rate of 13.98 MHz was clearly observed with a peak-to-background ratio of 67 dB (Figure 10d). The SA performance of WTe\({}_{2}\) microflakes was comparable to that of few-layered WTe\({}_{2}\) and its mode-locking performance was even better than that of few-layered WTe\({}_{2}\). Figure 10: Measured (**a**) oscilloscope trace, (**b**) optical spectrum, (**c**) autocorrelation trace and (**d**) electrical spectrum of the output optical pulses (SNR: signal-to-noise ratio, RBW: resolution bandwidth). Reprinted with permission from [33]. Figure 9: The schematic of the ring-cavity erbium-doped fiber laser incorporating the WTe\({}_{2}\) microflake-based saturable absorber (SA). Here, polarization controller (PC), Er-doped fiber (EDF), wavelength division multiplexing (WDM) and laser diode (LD) denote a polarization controller, an Er-doped fiber, a wavelength division multiplexing and a laser diode, respectively. Reprinted with permission from [33].