# High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development

^{1}

^{2}

^{*}

## Abstract

**:**

^{18}cm

^{−3}), which results in improved performance at telecom wavelengths (1560 nm). These materials have demonstrated a huge modulation depth of 23% and a saturation fluence of 830 µJ/cm

^{2}, and a large saturable absorption around −3 × 10

^{4}cm/GW has been observed, attaining an enhanced, nonlinear change in transmittance. We have studied the use of such InN layers as semiconductor saturable absorber mirrors (SESAMs) for an erbium (Er)-doped fiber laser to perform mode-locking generation at 1560 nm. We demonstrate highly stable, ultrashort (134 fs) pulses with an energy of up to 5.6 nJ.

## 1. Introduction

^{18}cm

^{−3}, which results in an absorption edge energy below 0.7 eV at room temperature [30]. It is hence interesting to study this material as a saturable absorber at 1560 nm.

^{4}cm

^{−1}and −3 × 10

^{4}cm/GW, respectively, a modulation depth of 23%, and saturation attained for a power density of 4 GW/cm

^{2}. This material was inserted in a fiber laser cavity, delivering pulses with an ultrashort pulse duration of 134.4 fs and energies of up to 5.6 nJ. These results make InN a promising candidate as a saturable absorber for ultrafast laser applications in the telecom range.

## 2. Material Characterization

^{3}for S1 and S2, 4–5 ${\times 10}^{18}$ cm

^{3}for S0′, and higher than ${10}^{19}$ cm

^{3}for S0 [33]. The smaller bandgap energy in S1 and S2 can be attributed to the reduction of the Burstein–Moss effect, due to a lower carrier concentration, and therefore to an improvement of the crystal quality because of a higher control of the growth conditions related to the time intervals of N deposition [34].

_{2}) for each sample. The SAs are excited by an ultrafast, mode-locked fiber laser with a pulse width of 250 fs, beam waist (${w}_{0}$) of 8.5 µm, and a Rayleigh distance (${z}_{R}$) of 223.2 µm (characterized by the knife-edge technique [36,37]), operating at 1560 nm with a repetition rate of 5.6 MHz. The transmitted signal is monitored by a Ge photodiode (Detector B, Thorlabs SM05PD6A) as the sample moves gradually along the propagation direction on a motorized translation stage. A power meter detects the output power of the reference beam in Detector A (Thorlabs PM100USB), in order to control possible oscillations of the laser. The laser beam has a maximum peak power of 40 kW, which corresponds to an energy fluence of 7 mJ/cm

^{2}incident on the sample surface.

_{ns}− T

_{lin}, was 23.2% for S1 and 17.3% for S2, while S0 and S0′ have modulation depths of 16.7% and 30.6%, respectively. A higher saturable absorption response was obtained for samples S1 and S2 compared to reference samples S0 and S0′, as shown in Figure 2b. The maximum nonlinear transmission change ΔT, calculated as the ratio between the non-saturable and linear transmission coefficients, was obtained for S1 (241%) and S2 (715%), which means at least a two-fold improvement for S0 and S0′. Note that the reduced transmission change in S1 with respect to S2 is due to the smaller InN layer thickness, and thus to a higher ${T}_{\mathrm{lin}}$ coefficient (16.4% for S1 and 2.8% for S2).

^{2}) without performing any apparent damage to the bulk InN saturable absorber [40], denoting the viability of these materials for high-power, ultrafast applications. This suggests promising results as saturable absorbers in ultrafast lasers.

## 3. Pulsed Laser Operation

^{2}/m. In this experiment, a variable optical attenuator is inserted to control the optical losses within the laser cavity. In order to maximize the optical power onto the sample, a 70/30 optical fiber coupler was included, so that 70% of the signal was recirculated inside the cavity, whereas the remaining 30% was the laser output. This configuration has demonstrated the best results for this type of laser cavity [29], in comparison to a transmitted configuration, as described in previous results [30]. The laser cavity had a total length of 38 m, from which 22 m corresponded to single-mode fiber (SMF) with anomalous dispersion (−0.021 ps

^{2}/m). It should be noticed that due to the bulk structure and small thickness, small Fabry–Pérot oscillations in the transmission measurement were measured for the InN semiconductor (inset of Figure 1), which yielded a negligible group-velocity dispersion (GVD) coefficient compared to the ones obtained for SMF and EDF. Thus, the laser cavity behaves as a dispersion-managed cavity [41,42], with a net dispersion coefficient of −0.21 ps

^{2}, operating in the anomalous dispersion regime.

_{AC}= 221 fs is the pulse duration of the autocorrelation function), and there is a 3 dB spectral bandwidth of 26.4 nm at 1564 nm. A 0.648 constant has been applied to transform the autocorrelation (AC) temporal width to the pulse profile, as in reference [44].

_{AC}= 190 fs), and the optical bandwidth at 3 dB decay was 40 nm, centered at a wavelength of 1569 nm. In this case, a more chirped pulse was obtained with TBP = 0.62.

## 4. Discussion

^{2}on the saturable absorber).

^{2}, which yields some limitations in the ultrafast laser energy per pulse. Fabrication methods that monitor material deposition and the controllable optical properties of 2D materials are still an outstanding goal. Table 2 summarizes the optical properties of the modified, InN-based saturable absorbers compared to some 2D materials in erbium-doped, mode-locked fiber lasers.

## 5. Conclusions

^{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 × 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.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

- Hirao, M.; Tsuji, S.; Mizuishi, K.; Doi, A.; Nakamura, M. Long wavelength InGaAsP/InP lasers for optical fiber communication systems. J. Opt. Commun.
**1980**, 1, 10–14. [Google Scholar] [CrossRef] - Li, Z.; Heidt, A.M.; Simakov, N.; Jung, Y.; Daniel, J.M.O.; Alam, S.U.; Richardson, D.J. Diode-pumped wideband thulium-doped fiber amplifiers for optical communications in the 1800–2050 nm window. Opt. Express
**2013**, 21, 26450–26455. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Xu, C.; Wise, F. Recent advances in fibre lasers for nonlinear microscopy. Nat. Photonics
**2013**, 7, 875–882. [Google Scholar] [CrossRef] - Van, V.; Ibrahim, T.A.; Absil, P.P.; Johnson, F.G.; Grover, R.; Ho, P.-T. Optical signal processing using nonlinear semiconductor microring resonators. J. Sel. Top. Quantum Electron.
**2002**, 8, 705–713. [Google Scholar] [CrossRef] - Gattass, R.; Mazur, E. Femtosecond laser micromachining in transparent materials. Nat. Photonics
**2008**, 2, 219–225. [Google Scholar] [CrossRef] - Toor, F.; Jackson, S.; Shang, X.; Arafin, S.; Yang, H. Mid-infrared Lasers for medical applications: Introduction to the feature issue. Biomed. Opt. Express
**2018**, 9, 6255–6257. [Google Scholar] [CrossRef] - Shirk, M.D.; Molian, P.A. A review of ultrashort pulsed laser ablation of materials. J. Laser Appl.
**1998**, 10, 18. [Google Scholar] [CrossRef] - Taccheo, S. Fiber lasers for medical diagnostics and treatments: State of the art, challenges and future perspectives. In Proceedings of the Optical Fibers and Sensors for Medical Diagnostics and Treatment Applications XVII, San Francisco, CA, USA, 23 March 2017. [Google Scholar]
- Dong, L.; Samson, B. Military applications of lasers. In Fiber Lasers: Basics, Technology, and Applications; CRC Press: Boca Raton, FL, USA, 2017; pp. 299–311. [Google Scholar]
- Anderberg, B.; Wolbarsht, M.L. Laser Weapons: The Dawn of a New Military Age, 1st ed.; Springer US Publisher: New York, NY, USA, 1992; p. 244. [Google Scholar]
- Garmire, E.; Yariv, A. Laser mode-locking with saturable absorbers. IEEE J. Quantum Electron.
**1967**, 3, 222–226. [Google Scholar] [CrossRef] - Keller, U. Recent developments in compact ultrafast lasers. Nature
**2003**, 424, 831–838. [Google Scholar] [CrossRef] - Keller, U.; Weingarten, K.J.; Kartner, F.X. Semiconductor saturable absorber mirrors (SESAM’s) for femtosecond to nanosecond pulse generation in solid-state lasers. IEEE J. Sel. Top. Quantum Electron.
**1996**, 2, 435–453. [Google Scholar] [CrossRef] [Green Version] - Haiml, M.; Grange, R.; Keller, U. Optical characterization of semiconductor saturable absorbers. Appl. Phys. B
**2004**, 79, 331–339. [Google Scholar] [CrossRef] [Green Version] - Popa, D.; Sun, Z.; Hasan, T.; Cho, W.B.; Wang, F.; Torrisi, F.; Ferrari, A.C. 74-fs nanotube-mode-locked fiber laser. Appl. Phys. Lett.
**2012**. [Google Scholar] [CrossRef] - Tang, D.Y.; Zhao, L.M. Generation of 47-fs directly from and erbium-doped fiber laser. Opt. Lett.
**2007**, 32, 41–43. [Google Scholar] [CrossRef] [PubMed] - Haefner, C.L.; Bayramian, A.; Betts, S.; Bopp, R.; Cupal, S.J.; Drouin, M.; Erlandson, A.; Horáček, J.; Horner, J.; Jarboe, J.; et al. High average power, diode pumped petawatt laser systems: A new generation of lasers enabling precision science and commercial applications. In Proceedings of the SPIE Research Using Extreme Light: Entering New Frontiers with Petawatt-Class Lasers III, Prague, Czech Republic, 26 June 2017. [Google Scholar]
- Zhang, B.; Liu, J.; Wang, C.; Yang, K.; Lee, C.; Zhang, H.; He, J. Recent progress in 2D material-based saturable absorbers for all solid-state pulsed bulk lasers. Laser Photonics Rev.
**2020**, 14, 1900240. [Google Scholar] [CrossRef] - Ahmad, H.; Safaei, R.; Rezayi, M.; Amiri, I.S. Novel D-shaped fiber fabrication method for saturable absorber application in the generation of ultra-short pulses. Laser Phys. Lett.
**2017**, 14, 085001. [Google Scholar] [CrossRef] - James, R.B.; Smith, D.L. Dependence of the saturation intensity of p-type germanium on impurity concentration and residual absorption at 10.59 μm. Solid State Commun.
**1980**, 33, 395–398. [Google Scholar] [CrossRef] - Tao, L.; Chen, K.; Chen, Z.; Chen, W.; Gui, X.; Chen, H.; Li, X.; Xu, J.B. Centimeter-Scale CVD growth of highly crystalline single-layer MoS
_{2}film with spatial homogeneity and the visualization of grain boundaries. ACS Appl. Mater. Interfaces**2017**, 9, 12073–12078. [Google Scholar] [CrossRef] - Potin, V.; Ruterana, P.; Nouet, G.; Pond, R.C.; Morkoç, H. Mosaic growth of GaN on (0001) sapphire: A high-resolution electron microscopy and crystallographic study of threading dislocations from low-angle to high-angle grain boundaries. Phys. Rev. B
**2000**, 61, 5587–5599. [Google Scholar] [CrossRef] - Ruterana, P.; Potin, V.; Barbaray, B.; Nouet, G. Growth defects in GaN layers on top of (0001) sapphire: A geometrical investigation of the misfit effect. Philos. Mag. A
**2000**, 80, 937–954. [Google Scholar] [CrossRef] - Ruterana, P.; Syrkin, A.L.; Monroy, E.; Valcheva, E.; Kirilov, K. The microstructure and properties of InN layers. Phys. Status Solidi C
**2010**, 7, 1301–1304. [Google Scholar] [CrossRef] - Monteagudo-Lerma, L.; Valdueza-Felip, S.; Núñez-Cascajero, A.; Ruiz, A.; González-Herráez, M.; Monroy, E.; Naranjo, F.B. Morphology and arrangement of InN nanocolumns deposited by radio-frequency sputtering: Effect of the buffer layer. J. Cryst. Growth
**2016**, 434, 13–18. [Google Scholar] [CrossRef] - Naranjo, F.B.; Kandaswamy, P.K.; Valdueza-Felip, S.; Calvo, V.; González-Herráez, M.; Martín-López, S.; Corredera, P.; Méndez, J.A.; Mutta, G.R.; Lacroix, B.; et al. Nonlinear absorption of InN/InGaN multiple-quantum-well structures at optical telecommunication wavelengths. Appl. Phys. Lett.
**2011**, 98, 031902. [Google Scholar] [CrossRef] [Green Version] - Bhuiyan, A.G.; Hashimoto, A.; Yamamoto, A. Indium nitride (InN): A review on growth, characterization, and properties. Appl. Phys.
**2003**, 94, 2779–2804. [Google Scholar] [CrossRef] - Wu, J.; Walukiewicz, W.; Yu, K.M.; Ager, J.W., III; Haller, E.E.; Lu, H.; Schaff, W.J.; Saito, Y.; Nanishi, Y. Unusual properties of the fundamental band gap of InN. Appl. Phys. Lett.
**2002**, 80, 3967. [Google Scholar] [CrossRef] - Jiménez-Rodríguez, M.; Monroy, E.; González-Herráez, M.; Naranjo, F.B. Ultrafast fiber laser using InN as saturable absorber mirror. J. Light. Technol.
**2018**, 36, 2175–2182. [Google Scholar] [CrossRef] - Monroy, L.; Jiménez-Rodríguez, M.; Ruterana, P.; Monroy, E.; González-Herráez, M.; Naranjo, F.B. Effect of the residual doping on the performance of InN epilayers as saturable absorbers for ultrafast lasers at 1.55 µm. Opt. Mater. Express
**2019**, 9, 2785–2792. [Google Scholar] [CrossRef] - Tauc, J. Optical properties and electronic structure of amorphous Ge and Si. Mater. Res. Bull.
**1968**, 3, 37–46. [Google Scholar] [CrossRef] - Viezbicke, B.D.; Patel, S.; Davis, B.E.; Birnie, D.P. Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system. Phys. Status Solidi B
**2015**, 252, 1700–1710. [Google Scholar] [CrossRef] - Wu, J.; Walukiewicz, W.; Li, S.X.; Armitage, R.; Ho, J.C.; Weber, E.R.; Haller, E.E.; Lu, H.; Schaff, W.J.; Barcz, A.; et al. Effects of electron concentration on the optical absorption edge of InN. Appl. Phys. Lett.
**2004**, 84, 2805–2807. [Google Scholar] [CrossRef] [Green Version] - Hsu, L.; Jones, R.E.; Li, S.X.; Yu, K.M.; Walukiewicz, W. Electron mobility in InN and III-alloys. J. Appl. Phys.
**2007**, 102, 1–6. [Google Scholar] [CrossRef] - Sheik-bahae, M.; Said, A.A.; Wei, T.H.; Hagan, D.J.; Vanstryland, E.W. Sensitive measurement of optical nonlinearities using a single beam. IEEE J. Sel. Top. Quantum Electron.
**1990**, 26, 760–769. [Google Scholar] [CrossRef] [Green Version] - Arnaud, J.A.; Hubbard, W.M.; Mandeville, G.D.; de la Clavière, B.; Franke, E.A.; Franke, J.M. Technique for fast measurement of gaussian laser beam parameters. Appl. Opt.
**1971**, 10, 2775–2776. [Google Scholar] [CrossRef] - de Araújo, M.A.; Silva, R.; de Lima, E.; Pereira, D.P.; de Oliveira, P.C. Measurement of Gaussian laser beam radius using the knife-edge technique: Improvement on data analysis. Appl. Opt.
**2009**, 48, 393–396. [Google Scholar] [CrossRef] - Chapple, P.B.; Staromlynska, J.; Hermann, J.A.; Mckay, T.J.; Mcduff, R.G. Single-Beam Z-scan: Measurement techniques and analysis. J. Nonlinear Opt. Phys. Mater.
**1997**, 6, 251–293. [Google Scholar] [CrossRef] - Abidin, M.S.; Muhammad, A.S.; Rashid, S.A.; Mahdi, M.A. Frequency and duty cycle modulation optimization in minimizing thermal accumulation effect in Z-scan measurement with high-repetition-rate laser. Jpn. J. Appl. Phys.
**2014**, 53. [Google Scholar] [CrossRef] [Green Version] - Gallazzi, F.; Jimenez-Rodriguez, M.; Monroy, E.; Corredera, P.; González-Herráez, M.; Naranjo, F.B.; Castañón, J.D.A. Megawatt peak-power femtosecond ultralong ring fibre laser with InN SESAM. In Proceedings of the Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, Munich, Germany, 23–27 June 2019. [Google Scholar]
- Tamura, K.; Ippen, E.P.; Haus, H.A.; Nelson, L.E. 77-fs pulse generation from a stretched-pulse mode-locked all-fiber ring laser. Opt. Lett.
**1993**, 18, 1080–1082. [Google Scholar] [CrossRef] - Turitsyn, S.K.; Bale, B.G.; Fedoruk, M.P. Dispersion-managed solitons in fibre systems and lasers. Phys. Rep.
**2012**, 521, 135–203. [Google Scholar] [CrossRef] - Jiménez-Rodríguez, M.; Monteagudo-Lerma, L.; Monroy, E.; González-Herráez, M.; Naranjo, F.B. Widely power-tunable polarization-independent ultrafast mode-locked fiber laser using bulk InN as saturable absorber. Opt. Express
**2017**, 25, 5366–5375. [Google Scholar] [CrossRef] [Green Version] - Diels, J.M.; Fontaine, J.J.; McMichael, I.A.; Simoni, F. Control and measurement of ultrashort pulse shapes (in amplitude and phase) with femtosecond accuracy. Appl. Opt.
**1985**, 24, 1270–1282. [Google Scholar] [CrossRef] - Campagnola, P.J.; Wei, M.; Lewis, A.; Loew, L.M. High-Resolution nonlinear optical imaging of live cells by second harmonic generation. Biophys. J.
**1999**, 77, 3341–3349. [Google Scholar] [CrossRef] [Green Version] - Rizvi, N.H. Femtosecond laser micromachining: Current status and applications. Riken Rev.
**2003**, 50, 1–10. [Google Scholar] - Woodward, R.I.; Kelleher, E.J.R. 2D saturable absorbers for fibre lasers. Appl. Sci.
**2015**, 5, 1440–1456. [Google Scholar] [CrossRef] [Green Version] - Liu, X.; Gao, Q.; Zheng, Y.; Mao, D.; Zhao, J. Recent progress of pulsed fiber lasers based on transition-metal dichalcogenides and black phosphorus saturable absorbers. Nanophotonics
**2020**. [Google Scholar] [CrossRef] [Green Version] - Jiang, T.; Yin, K.; Wang, C.; You, J.; Ouyang, H.; Miao, R.; Zhang, C.; Wei, K.; Li, H.; Chen, H.; et al. Ultrafast fiber lasers mode-locked by two-dimensional materials: Review and prospect. Photonics Res.
**2020**, 8, 78–90. [Google Scholar] [CrossRef] - Autere, A.; Jussila, H.; Dai, Y.; Wang, Y.; Lipsanen, H.; Sun, Z. Nonlinear optics with 2D layered materials. Adv. Mater.
**2018**, 30. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Eibl, M.; Weng, D.; Hakert, H.; Kolb, J.P.; Pfeiffer, T.; Hundt, J.E.; Huber, R.; Karpf, S. Wavelength agile multi-photon microscopy with a fiber amplified diode laser. Biomed. Opt. Express
**2018**, 9, 6273–6282. [Google Scholar] [CrossRef] - Vogel, A.; Venugopalan, V. Mechanisms of pulsed laser ablation of biological tissues. Chem. Rev.
**2003**, 103, 577–644. [Google Scholar] [CrossRef] [Green Version] - Zhang, W.; Wang, O.; Chen, Y.; Wang, Z.; Wee, A.T.S. Van der Waals stacked 2D layered materials for optoelectronics. 2D Mater.
**2016**, 3. [Google Scholar] [CrossRef] - Bao, Q.; Zhang, H.; Wang, Y.; Ni, Z.; Yan, Y.; Shen, Z.X.; Loh, K.P.; Tang, D.Y. Atomic-layer graphene as a saturable absorber for ultrafast pulsed lasers. Adv. Funct. Mater.
**2009**, 19, 3077–3083. [Google Scholar] [CrossRef] - Sotor, J.; Pasternak, I.; Krajewska, A.; Strupinski, W.; Sobon, G. Sub-90 fs a stretched-pulse mode-locked fiber laser based on a graphene saturable absorber. Opt. Express
**2015**, 23, 27503–27508. [Google Scholar] [CrossRef] - Yan, P.; Lin, R.; Ruan, S.; Liu, A.; Chen, H.; Zheng, Y.; Chen, S.; Guo, C.; Hu, J. A practical topological insulator saturable absorber for mode-locked fiber laser. Sci Rep.
**2005**, 5, 8690. [Google Scholar] [CrossRef] - Liu, H.; Zheng, X.; Liu, M.; Zhao, N.; Luo, A.; Luo, Z.; Xu, W.; Zhang, H.; Zhao, C.; Wen, S. Femtosecond pulse generation from a topological insulator mode-locked fiber laser. Opt. Express
**2014**, 22, 6868–6873. [Google Scholar] [CrossRef] [PubMed] [Green Version] - Koo, J.; Park, J.; Lee, J.; Jhon, Y.M.; Lee, J.H. Femtosecond harmonic mode-locking of a fiber laser at 3.27 GHz using a bulk-like, MoSe
_{2}-based saturable absorber. Opt. Express**2016**, 24, 10575–10589. [Google Scholar] [CrossRef] [PubMed] - Liu, W.J.; Liu, M.L.; Yang, Y.; Hou, H.; Ma, G.; Lei, M.; Wei, Z.Y. Tungsten diselenide for mode-locked erbium-doped fiber lasers with short pulse duration. Nanotechnology
**2018**, 29, 1–12. [Google Scholar] [CrossRef] - Liu, W.J.; Liu, M.L.; Liu, B.; Quhe, R.G.; Lei, M.; Fang, S.B.; Teng, H.; Wei, Z.Y. Nonlinear optical properties of MoS
_{2}-WS_{2}heterostructure in fiber lasers. Opt. Express**2019**, 27, 6689–6699. [Google Scholar] [CrossRef] - Sotor, J.; Sobon, G.; Macherzynski, W.; Paletko, P.; Abramski, K.M. Black phosphorus saturable absorber for ultrashort pulse generation. Appl. Phys. Lett.
**2015**, 107, 051108. [Google Scholar] [CrossRef] [Green Version] - Jin, X.; Hu, G.; Zhang, M.; Hu, Y.; Albrow-Owen, T.; Howe, R.C.T.; Wu, T.; Wu, Q.; Zheng, Z.; Hasan, T. 102 fs pulse generation from a long-term stable, inkjet-printed black phosphorus-mode-locked fiber laser. Opt. Express
**2018**, 26, 12506–12513. [Google Scholar] [CrossRef] - Jiang, X.; Liu, S.; Liang, W.; Luo, S.; He, Z.; Ge, Y.; Wang, H.; Cao, R.; Zhang, F.; Wen, Q.; et al. Broadband nonlinear photonics in few-layer MXene Ti
_{3}C_{2}T_{x}(T = F, O, or OH). Laser Photonics Rev.**2018**, 12, 1–10. [Google Scholar] - Guo, B.; Wang, S.; Wu, Z.; Wang, Z.; Wang, D.; Huang, H.; Zhang, F.; Ge, Y.; Zhang, H. Sub-200 fs soliton mode-locked fiber laser based on bismuthene saturable absorber. Opt. Express
**2018**, 26, 22750–22760. [Google Scholar] [CrossRef] - Pumera, M.; Sofer, Z. 2D monoelemental arsenene, antimonene, and bismuthene: Beyond black phosphorus. Adv. Mater.
**2017**, 29, 1605299. [Google Scholar] [CrossRef]

**Figure 1.**Tauc’s plot for direct electronic transitions, i.e., representation of (αE)

^{2}versus energy (E), for samples S0 (

**black line**), S0′ (

**green line**), S1 (

**blue line**), and S2 (

**red line**). The vertical dashed line indicates the operation wavelength of the laser. The inset shows the transmission of each sample as a function of the incident wavelength.

**Figure 2.**(

**a**) Schematic description of the setup for Z-scan measurements. (

**b**) Normalized nonlinear transmission change as a function of the focal distance for the case of maximum incident pulse energy (40 kW). (

**c**) Evolution of the nonlinear transmittance versus the incident pulse fluence at 1560 nm. Solid lines are fitting curves to Equation (2).

**Figure 3.**(

**a**) Schematic set-up of the erbium (Er)-doped, mode-locked fiber laser, using InN as a saturable absorber. (

**b**) Oscilloscope trace for fundamental mode-locking operations at 5.6 MHz.

**Figure 4.**(

**a**) Mode-locked autocorrelation trace for reference sample (S0′) and modified saturable absorbers (SAs) (S1 and S2) with a Gaussian fitting function, i.e., the pulse duration of the autocorrelation function (τ

_{AC}) = 235 fs for S0′, τ

_{AC}= 221 fs for S1, and τ

_{AC}= 190 fs for S2. (

**b**) Optical spectrum of each sample centered in 1560 nm and fitted to a Gaussian function (22.6 nm for reference sample S0′, 25.4 nm for S1, and 40 nm for S2), with a pump power of 70 mW within the laser cavity.

**Table 1.**Nonlinear optical parameters calculated from the fitting formula (2) to the experimental data for the different samples.

Sample | ${\mathit{F}}_{\mathbf{sat}}\text{}({\mu}\mathbf{J}/{\mathbf{cm}}^{2})$ | ${\mathit{T}}_{\mathbf{lin}}\text{}(\%)$ | ${\mathit{T}}_{\mathbf{ns}}\text{}(\%)$ | $\Delta \mathit{T}\text{}(\%){}^{1}$ |
---|---|---|---|---|

S0 | 357.2 | 27.8 | 44.5 | 160 |

S0′ | 383.8 | 12.7 | 43.3 | 340 |

S1 | 756.6 | 16.4 | 39.6 | 241 |

S2 | 831.5 | 2.81 | 20.1 | 715 |

^{1}ΔT has been calculated as 100 × ${T}_{\mathrm{ns}}/{T}_{\mathrm{lin}}$.

**Table 2.**Optical properties of mode-locked, erbium-doped fiber lasers, using two-dimensional (2D) materials as saturable absorbers (SA) at telecom wavelengths.

SA | λ (nm) | Δλ (nm) | τ_{pulse} (fs) | MD (%) ^{1} | I_{sat} (MW/cm^{2}) ^{2} | P_{p} (kW) ^{3} | Ref. |
---|---|---|---|---|---|---|---|

Graphene | 1545 | 48 | 88 | 11 | 2 × 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 × 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 × 10^{3} | 22.3 | [29] |

InN | 1569 | 40 | 134.4 | 17.3 | 4.4 × 10^{3} | 28.2 | This work |

^{1}MD corresponds to the modulation depth coefficient.

^{2}

**I**represents the saturated intensity.

_{sat}^{3}P

_{p}represents the peak power of the optical pulse.

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Monroy, L.; Jiménez-Rodríguez, M.; Monroy, E.; González-Herráez, M.; B. Naranjo, F.
High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development. *Appl. Sci.* **2020**, *10*, 7832.
https://doi.org/10.3390/app10217832

**AMA Style**

Monroy L, Jiménez-Rodríguez M, Monroy E, González-Herráez M, B. Naranjo F.
High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development. *Applied Sciences*. 2020; 10(21):7832.
https://doi.org/10.3390/app10217832

**Chicago/Turabian Style**

Monroy, Laura, Marco Jiménez-Rodríguez, Eva Monroy, Miguel González-Herráez, and Fernando B. Naranjo.
2020. "High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development" *Applied Sciences* 10, no. 21: 7832.
https://doi.org/10.3390/app10217832