# High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development
## 5 Conclusions  
In summary, reducing the residual carrier concentration in InN below 10\({}^{18}\) cm\({}^{-3}\) results in an enhancement of the nonlinear absorption at 1560 nm. Thus, we have demonstrated an InN epitaxial layer, with a thickness of 780 nm, that displays a saturable intensity of 4.4 GW/cm\({}^{2}\), a modulation depth of 17.3%, and a nonlinear absorption coefficient in up to \(-3\times 10^{4}\) cm/GW, and which has been observed attaining an enhanced nonlinear change in transmittance (715% of transmission change). This suggests promising results as saturable absorbers in ultrafast lasers. Based on this saturable absorber, an ultrafast fiber laser was developed with a pulse duration of 134.4 fs and peak power up to 28.2 kW. The InN-based SESAM has demonstrated not only some advantages in terms of ease of fabrication, robustness, and crystalline quality, but also a higher achievable peak power in comparison with other materials. These results make InN a promising candidate for the production of commercial saturable absorbers in the telecom spectral range.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**SA** & **\(\lambda\) (nm)** & **\(\Delta\lambda\) (nm)** & \(\tau_{\text{pulse}}\) (fs)** & **MD (\%) 1** & \(I_{\text{sat}}\) (MW/cm2) 2 & \(P_{\text{P}}\) (kW) 3 & **Ref.** \\ \hline Graphene & 1545 & 48 & 88 & 11 & \(2\times 10^{3}\) & 0.57 & [55] \\ TI–Bi\({}_{2}\)Se\({}_{3}\) & 1557.5 & 4.3 & 660 & 3.9 & 12 & 0.22 & [57] \\ TMD–WSe\({}_{2}\) & 1557.4 & 25.8 & 163.5 & 21.9 & 15.4 & 1.79 & [59] \\ TMD–MoS\({}_{2}\) & 1560 & 24.4 & 154 & 19.12 & 1.361 & 1.12 & [60] \\ BP & 1555 & 40 & 102 & 10 & 15 & 0.49 & [62] \\ MXene & 1555 & 22.2 & 159 & 24 & \(39.1\times 10^{3}\) & 2.6 & [63] \\ Bismuthene & 1561 & 14.4 & 193 & 5.6 & 48.2 & 3.28 & [64] \\ InN & 1562 & 22.6 & 166.2 & 30.6 & \(1.6\times 10^{3}\) & 22.3 & [29] \\ InN & 1569 & 40 & 134.4 & 17.3 & \(4.4\times 10^{3}\) & 28.2 & This work \\ \hline \hline \end{tabular} \({}^{1}\) MD corresponds to the modulation depth coefficient. 2 \(I_{\text{sat}}\) represents the saturated intensity. 3 \(P_{\text{P}}\) represents the peak power of the optical pulse.  
\end{table}
Table 2: Optical properties of mode-locked, erbium-doped fiber lasers, using two-dimensional (2D) materials as saturable absorbers (SA) at telecom wavelengths.  
**Author Contributions:** Material development, E.M.; formal analysis, L.M.; investigation, L.M., M.J.-R. and F.B.N.; data curation, L.M. and M.J.-R.; writing--original draft preparation, L.M.; writing--review and editing, E.M., M.G.-H. and F.B.N.; supervision, E.M., M.G.-H. and F.B.N. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by Comunidad de Madrid, grant number S2018/NMT-4326; by the Ministerio de Ciencia, Innovacion y Universidades, grant number RTI2018-097957-B-C31; and the Ministerio de Economia, Industria y Competitividad, Gobierno de Espana, with grant number TEC2015-71127-C2-2-R.  
**Acknowledgments:** Authors want to thank [PERSON] from Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT) for the transmittance measurements.  
**Conflicts of Interest:** The authors declare no conflict of interest.