# Saturable Absorption Dynamics of Highly Stacked 2D Materials for Ultrafast Pulsed Laser Production
## 6 Perspective of 2D Material SAs  
Through extensive researches, it was observed that the mode-locking performance of 2D material SAs was seldom deteriorated by significant layer stacking as summarized in Table 1 including studies introduced in this review. We see that femtosecond pulsed lasers can be readily produced by high-stacked 2D material SAs. Figure 21: (**a**) Oscilloscope traces of output pulses measured at various pump powers. (**b**) Optical spectrum of output pulses obtained at a pump power of 273 mW. Reprinted with permission from [80]. Figure 20: The configuration of a Q-switched fiber laser using a pencil graphite SA. Reprinted with permission from [80]. Apart from sandwiched and/or side-polished fiber modules, SAs can also be incorporated into tapering fiber modules in which coating is etched radially around the fiber and SAs are deposited all around the etched fiber. Highly stacked SAs still showed excellent modulation performance in this type of SA modules [83]. In this review, we only introduced the progresses of 2D material SAs achieved in the spectral range of fiber photonics. However, using 2D materials with appropriate electronic and optical properties, the application range can be further extended to other spectral regions such as visible and terahertz ones [84; 85]. Visible orange-light laser pulses were readily produced using a WS\({}_{2}\)-coated mirror and a Pr\({}^{3+}\)-doped ZBLAN (ZrF\({}_{4}\)-BaF\({}_{2}\)-LaF\({}_{3}\)-AlF\({}_{3}\)-NaF) fiber gain medium (core/cladding: 6/125 \(\upmu\)m, numerical aperture (NA) of 0.15, Pr\({}^{3+}\) concentration of 1000), suggesting a possible broadband application of 2D material SAs [84]. The setup for this free-space pulsed laser is shown in Figure 22. When a WS\({}_{2}\) SA was not used in the system, it generated orange-light continuous wave lasers. However, when WS\({}_{2}\) mirrors were incorporated into the laser system, it produced orange-light Q-switched lasers whose typical oscilloscopic traces were clearly observed for various input powers (Figure 23). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Saturable** & **Fiber Platform** & **Wave Length (nm)** & **Modulation** & **Saturation Level** & **Pulse Width (fs)** & **Refs** \\ \hline Few Layered WT\({}_{2}\) & Side-Polished Fiber & 1561 & 2.8 & NA & 1200 & [49] \\ \hline Bulk Structured WT\({}_{2}\) & Side-Polished Fiber & 1556.2 & NA & 59 MW/cm\({}^{2}\) & 770 & [33] \\ \hline Nanostructured Bi\({}_{2}\)Te\({}_{3}\) & Tapered Fiber & 1909.5 & 9.8 & NA & 1260 & [82] \\ \hline Bulk Structured Bi\({}_{2}\)Te\({}_{3}\) & Side-Polished Fiber & 1935 & 20.6 & 29 W & 795 & [66] \\ \hline Graphene & Fiber Ferrule & 1884 & NA & NA & 1200 & [79] \\ \hline Graphene & Fiber Ferrule & 1555 & 3.6 & 0.08 & 590 & [34] \\ \hline Graphite & Fiber Ferrule & 1563 & 21 & NA & 472 & [81] \\ \hline \hline \end{tabular}  
NA: Not Available. Saturation level units of W and MW/cm\({}^{2}\) are for saturation power and saturation intensity, respectively.  
\end{table} 
Table 1: Output performance comparison of mode-locked fiber lasers using highly stacked 2D material SAs and others. Figure 22: (**a**) Schematic illustration and (**b**) experimental picture of compact transition metal dichalcogenides (TMDCs)-based passively Q-switched orange-light Pr\({}^{3+}\)-doped all-fiber laser. Transmission spectrum of the fiber pigtail mirrors (FPMs): (**c**) input mirror M1 and (**d**) output mirror M2. (**e**) An image of the FPM, and (**f**) a photograph of the fiber-end coated by a TMDC film. Reprinted with permission from [84]. Although highly stacked 2D material SAs can have a fairly good modulation performance in laser photonics, mono- and/or few-layer 2D material SAs would have their own merits as it could be more desirable for transmission-mode applications such as sandwiched-type SA modules in fiber laser systems and/or SA coated mirrors in free space lasers.