# Saturable Absorption Dynamics of Highly Stacked 2D Materials for Ultrafast Pulsed Laser Production
## 4 Topological Insulator SAs  
### Highly Stacked Bi\({}_{2}\)Te\({}_{3}\) SAs  
Mid-infrared laser technology has become of great research interest due to its promising potential in various emerging application fields such as mechanical processing [56], chemical detection [57], free space optical communication [58], LIDAR sensing [59; 60] and laser-assisted medical diagnostics and/or surgery [61; 62]. Recently, in addition to traditional mid-infrared technologies based on free-space lasers [63], considerable attention has been paid to the development of optical fiber-based mid-infrared laser systems because of their outstanding performances in terms of beam quality, compactness and reliability [64]. So far, optical fibers doped with transition metal ions such as thulium (Tm), holmium (Ho) and erbium (Er) ions have been used to make fiberized laser systems operating at the spectral region between 1.5 and 3.0 \(\upmu\)m, together with an appropriate selection of host materials such as silica, silicate, tellurite and fluoride glass. Fiber lasers can operate either in a continuous-wave or pulsed form, depending on the application purpose. For pulsed lasers, additional optical component should be implemented to induce Q-switching or mode-locking. Q-switching is achieved by modulating the quality factor (Q-factor) of the cavity in a transient mode, while mode-locking occurs by keeping the relative phases of multiple laser modes. Generally, these modulations can be achieved by two different approaches, passive and active, and SAs are the most important ingredient of passive modulation. Recently, 2D TIs have gained huge scientific and technical attention for the development of SAs suitable for fiber lasers, particularly at the midinfrared applications, due to their prominent properties and exotic surface electronic states [65]. However, complicated processes for preparation without layer stacking had deteriorated the practical application of 2D TI-based photonic devices. If layer stacking is not a critical factor for the proper SA operation of TIs, the applicability of TIs in this field would be greatly enhanced than otherwise, and this issue has been systematically addressed as described next [66]. For this study, a side-polished fiber module deposited with highly stacked Bi\({}_{2}\)Te\({}_{3}\) TIs were fabricated and its nonlinear optical transmission behavior was investigated in the evanescent regime using a 1-ps mode-locked fiber pulse laser at 1950 nm. The modulation depth of highly stacked Bi\({}_{2}\)Te\({}_{3}\) TIs was as large as 20.6%, while its saturation power was 29 W. Based on this fact, passively mode-locked laser system was constructed using the highly stacked Bi\({}_{2}\)Te\({}_{3}\) SA fiber module (Figure 11). Specifically, the ring cavity was made using a gain medium of a 1-m long Tm/Ho co-doped fiber, an isolator, a 90:10 coupler, a polarization controller, a 1550/2000-nm wavelength division multiplexing (WDM) and a Bi\({}_{2}\)Te\({}_{3}\)-deposited side-polished fiber module. For input of pump beam in the ring cavity, a 1550-nm semiconductor laser diode (LD) with a maximum power of 250 mW was coupled with the Tm/Ho co-doped fiber using a 1550/2000 nm WDM coupler. The mode-locked laser output was extracted through a 10% output port of a 90:10 coupler. A single-mode fiber of SM2000, which has a cut-off wavelength of 1700 nm, is known to be more suitable to the 2000-nm operation than a standard SMF28 fiber due to a significant lower bending loss, while keeping almost similar propagation loss and dispersion as those of SMF28. Hence, in this study, all the components were fusion-spliced using the SM2000 for easy handing of system fabrication and operation, and the total length of the ring cavity was about 7 m. The dispersion of the SM2000 and Tm/Ho codoped fiber were \(-0.067\) ps\({}^{2}\)/m and \(-0.056\) ps\({}^{2}\)/m, respectively, at a wavelength of 1950 nm. Based on these measurements, the total cavity dispersion was approximated as \(-0.458\) ps\({}^{2}\).