# High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development
## 4 Discussion  
The above-described results show that the reduced carrier concentration in the InN layers leads to an improvement of their performance as saturable absorbers. An enhancement of the output peak power and the energy per optical pulse (28.2 kW and 5.4 nJ) has been observed compared to the reference sample S0\({}^{\prime}\) (22.3 kW and 5.2 nJ). Also, the pulse duration is shorter (134.4 fs) than that obtained for S0\({}^{\prime}\) (166.2 fs) [29]. The reduction of the pulse duration, and thus the widening in the optical frequency domain, may be associated with a higher saturable absorption coefficient in S1 and S2. The enhanced nonlinear response can induce high-order nonlinear effects in the optical pulse, generating chirped pulses, which could also explain the spectral redshift in S1 and S2 observed in Figure 4b. The difference between samples S1 and S2 in terms of pulse duration and optical bandwidth is related to the variation in the active layer thickness [25]. On the other hand, a higher optical fluence has to be employed to achieve the optical bleaching of samples S1 and S2, expanding the energy operation range and thus denoting the viability of these materials for high-power applications in the telecom range. None of these samples has displayed any sign of optical damage, even for the maximum intracavity intensity (31.6 mW of average output power implicates 73.7 mW in the resonator and a fluence of 10.5 mJ/cm\({}^{2}\) on the saturable absorber). In this work, a pulse duration of 134.4 fs (obtained directly from the autocorrelation trace) and an optical bandwidth of 40 nm was obtained for the best performance (S2), which is among the shortest values compared to similar fiber laser saturable absorbers. The large nonlinear effects and the high damage threshold level of the optimized, InN-based semiconductors presented above make InN SAs a promising candidate for ultrafast laser applications, such as biophotonic imaging [45] or surface micromachining [46], among others. New types of saturable absorbers with high efficiency, compactness, and easy fabrication are currently being investigated for pulsed laser generation [47; 48; 49]. These saturable absorbers, also denoted as two-dimensional (2D) materials, are constituted by a few layers of atoms grown in one direction while keeping the crystal quality. In this regard, 2D materials have exhibited great results in nonlinear optics [50; 51], biomedicine [52], and optoelectronics [53], due to their dimensionality and optical properties. In this sense, graphene [54; 55], topological insulators (TIs) [56; 57], transition metal dichalcogenides (TMDs) [58; 59; 60], and black phosphorus [61; 62], are the most studied 2D materials for ultrafast fiber laser generation. Recently, some 2D materials, such as MXene [63], bismuthene [64], or antimonene [65] have emerged due to its attractive physical properties, allowing ultrafast recoveries,broad bandwidths, or low saturation fluences. However, the development of novel 2D materials with promising nonlinear optical properties is still a challenge, i.e., presenting damage thresholds below the 2 mJ/cm\({}^{2}\), 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. The results obtained in this work are comparable to those obtained for black phosphorus [61], bismuthene [64], or graphene [55], which have reported the narrowest pulse in a dispersion compensation fiber-ring cavity. Nevertheless, due to the limitations in the modulation depth coefficient, some new materials are being investigated, such as topological insulators, transition metal dichalcogenides (TMDs), and MXenes.