Stable ultrafast fiber lasers have become indispensable tools for a variety of applications, such as optical communication, spectroscopy, biomedical diagnosis, and industrial fabrication, for their excellent beam quality, efficient heat dissipation, compact configuration, and high-quality pulse generation ^[1–4]. To constitute a compact fiber laser, the saturable absorber (SA) plays a very important role in modulating laser systems to deliver an ultrafast laser. Inspired by the requirement of a stable, cost-effective ultrafast fiber laser, researchers from different communities try their best to develop novel SAs ^[5–7]. With the evolution of low-dimensional optical materials, graphene and other two-dimensional (2D) materials have become a hot research topic in recent years for their unique performance in the field of broadband optoelectronic devices resulting from their special electronic structure and excellent physical and chemical properties ^[8–14]. However, the lack of a well-controlled, stable preparation method remains a hurdle to applications of 2D materials and their optical properties; in particular, nonlinear optics still requires deeper understanding.

Recently, MXenes, a new series of 2D materials composed of early transition metal carbides and/or carbonitrides, was first introduced by Gogotsi’s group in 2011 ^[15]. As a new family of 2D materials, MXenes have drawn extensive attention in many areas, such as energy storage, electromagnetic interference shielding, composite reinforcement, water purification, chemical catalysts, biosensors, photothermal therapy, and light-to-heat conversion for energy harvesting ^[16–19]. Delamination of MXenes produces single-layer nanosheets with thicknesses of about a nanometer and lateral sizes on the order of micrometers. Their general formula is Mn+1XnTx (n=1–3) ^[20], where M represents an early transition metal (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, or Mo), X is carbon and/or nitrogen, and Tx stands for the surface terminations (e.g., hydroxyl, oxygen, or fluorine). Compared with the typical 2D materials such as graphene and molybdenum sulfide, MXenes have the advantages of good hydrophilicity, metallic conductivity, and adjustable chemical composition ^[21,22]. When it comes to the optical regime, some theoretical and experimental works have been reported ^[23–25]. However, the optical nonlinearities and related applications of MXenes have been paid less attention.

Recently, Jhon et al. investigated the potential of MXene compounds for producing Q-switched and/or mode-locked laser pulses through a combined experimental and computational approach in which Ti3CNTx was used as a MXene-based SA ^[26]. Jiang et al. measured the third-order susceptibilities of Ti3C2Tx film by a Z-scan technique in the broadband range, and achieved mode-locked fiber lasers at 1066 nm and 1555 nm ^[27]. Tuo et al. found that 2D transition metal carbides α–Mo2C crystals have excellent saturable absorption properties in terms of largely tunable modulation depth and very low saturation intensity ^[28]. Dong et al. experimentally found that Ti3C2Tx has superior tunable saturable absorption properties, and MXenes are more resilient than other 2D materials with high damage thresholds ^[29].

In this work, we have synthesized Ti3C2Tx (T = F, O, or OH) through an etching method. In addition, we have studied the ultrafast dynamics of the materials by transient absorption techniques, and investigated the nonlinear saturable absorption behavior of Ti3C2Tx film by using the open aperture Z-scan technique from ultraviolet 400 nm to an optical communication window around 1550 nm. In addition, the stable ultrafast fiber laser has been delivered successfully by inserting the Ti3C2Tx SA into an erbium-doped fiber laser cavity, with pulse duration down to 946 fs and signal-to-noise ratio (SNR) up to 70.7 dB at the central wavelength of 1567.3 nm.

MXene Ti3C2Tx was prepared on the basis of the method reported by Naguib et al. ^[15]. In a typical process, Ti3AlC2 powders (≥98% purity; Beijing Lianlixin Technology Co., Ltd., China) were immersed in aqueous hydrogen fluoride (HF) solution (40%) for 48 h at room temperature. After washing with deionized water several times, the resulting deposition was dried in vacuum at 60°C for 48 h. As shown in Fig. 1(a), the X-ray diffraction (XRD) pattern validated the successful fabrication of a pure Ti3AlC2 phase, which is in accordance with the standard spectrum (JCPDS 52-0875) ^[30].

After HF etching, the strongest XRD peak, e.g., (104), in the out-of-plane direction was significantly weakened, and a broader peak of (002) corresponding to in-plane diffraction appeared at a 2θ angle of 8.9° in Ti3C2Tx, indicating substantial expansion of the interlayer spacing from 9.3 Å (1 Å = 0.1 nm) for Ti3AlC2 to 9.9 Å for Ti3C2Tx ^[15]. The linear absorption of the Ti3C2Tx sample is characterized by the spectrometer (Shimadzu ISR-603) in Fig. 1(b), which shows that Ti3C2Tx has a big and relatively flat absorption from ultraviolet to mid-infrared. Scanning electron microscopy (SEM) shows that the material has a layered structure, and the exfoliated layer is relatively large, as shown in Figs. 1(c) and 1(d). In Fig. 1(e), the interlayer distance is determined as 9.85 Å, matching well with the previous theoretical value (9.93 Å) ^[31]. As shown in Fig. 1(f), the transmission electron microscope (TEM) analysis of the exfoliated nanosheets shows that they are quite thin and transparent to electrons.

We have characterized the ultrafast dynamics of the Ti3C2Tx via the femtosecond transient absorption spectrum, which is very important for potential ultrafast applications ^[32,33]. During the measurement, the 400 nm laser acts as the pump light, and the super continuum white light acts as the probe light. With the help of the time delay line, the transient absorption curves can be obtained, as shown in Fig. 2. The change in the absorption intensity obtained by the experiment is represented by the change in optical density ΔOD=lgInopumpIpump, where Inopump is the intensity of light detected when there is no pump light, and Ipump is the intensity of light detected when the pump light is on. In Fig. 2(a), the signal appears negative for a long time, and the valley around 650 nm shows no significant shift with time delay, exhibiting mainly saturable absorption. We have extracted the photodynamic curve of the sample at 650 nm from the transient absorption curve, as shown in Fig. 2(b). By fitting the experimental results ^[34], two relaxation times can be obtained. After the pump laser pulse is applied, the curve drops and recovers rapidly, with a lifetime of 0.81 ps, and then stabilizes slowly after 28.8 ps. The ultrafast recovery time makes Ti3C2Tx a promising nonlinear optical material for ultrafast applications, particularly as an effective SA in a mode-locked fiber laser.

To investigate the broadband nonlinear optical properties, a Ti3C2Tx-methanol solution was spin-coated onto 1 mm thick quartz glass. Then we experimentally investigated the saturable absorption property of the Ti3C2Tx using the open aperture Z-scan technique. The experimental setup is shown in Fig. 3. The ultrafast pulses have been delivered from two lasers (laser 1: center wavelength: 800 nm, pulse duration: 100 fs, 3 dB spectral width: 15 nm, and repetition rate: 1 kHz; laser II: center wavelength: 1560 nm, pulse duration: 100 fs, 3 dB spectral width: 10 nm, and repetition rate: 80 MHz). By frequency doubling, the experimental irradiation source adopted can cover the ultraviolet to near-infrared band. During the experiment, the 5:5 beam splitter can divide the laser beam into two paths. One beam is collected by detector 1 as a reference beam, and the other beam passes through the Ti3C2Tx sample with a focal length of 500 mm and is collected by detector 2. According to the CS2 measurement, the incident beam waist is fitted to be about 55 μm. During the experiment, the sample surface is perpendicular to the incident laser beam and moves back and forth around the beam focus.

The open aperture Z-scan measurements with the Ti3C2Tx sample are shown in Fig. 4. When the Ti3C2Tx sample approaches the beam focus, the normalized transmittance gradually increases. This shows that as the incident pump intensity increases, the light absorption of the sample becomes smaller and gradually becomes saturated. By using optical attenuators, the average power could be deliberately controlled. As shown in Figs. 4(a), 4(c), and 4(e), the peaks of the open-aperture Z-scan curves increase with the increase of the input peak intensity.

In Figs. 4(b), 4(d), and 4(f), we fit the experimental data with a typical nonlinear transmission function of an SA depending on the incident light intensity. The transmittance T has a relationship with input optical intensity I as ^[35] where αs is the modulation depth, αns is the non-saturable components, and Isat is the saturable intensity. The experimental data match quite well with Eq. (1). By fitting the experimental results, it was found that the modulation depth and saturable intensity are about 20% and 3.7GW/cm2 at 400 nm, 29% and 25GW/cm2 at 800 nm, and 41% and 7.3MW/cm2 at 1560 nm, respectively. The layered Ti3C2Tx nanosheets exhibited properties ranging from metals to semiconductors due to the different surface groups ^[15]. The experimental results show that in the optical communication band where the photon energy is relatively small, the nonlinear absorption exhibits a large modulation depth ^[36]. By taking advantage of the saturable absorption property, Ti3C2Tx could be developed as broadband passive nonlinear optical modulators ^[1].

With a fiber laser configuration with a cavity length of 24.8 m, as shown in Fig. 5, self-started mode-locking operation with a threshold pump of 55 mW has been obtained. In the laser cavity, we used a piece of 0.9 m erbium-doped fiber with a group velocity dispersion (GVD) parameter of 15.8 ps/(nm·km) and 23.9 m standard single-mode fiber with a GVD parameter of 18 ps/(nm·km) at 1550 nm. The pump from a 975 nm laser diode (LD) source was coupled into the cavity through a 980/1550 wavelength-division multiplexer (WDM), and a 10% fiber coupler was employed to deliver the laser emission. We incorporated the Ti3C2Tx-based side-polished fiber (SPF) that has a modulation depth of 41% and a saturable intensity of 7.3MW/cm2 in the fiber cavity. Figure 6 summarizes the characteristics of the mode-locking operation of the fiber laser at a pump power of 60 mW. Figure 6(a) shows the single-pulse train recorded over 2 μs. The pulses have a repetition rate of 8.24 MHz, which matches well with the cavity length, indicating that the laser operates in the mode-locking state. The corresponding optical spectrum, as shown in Fig. 6(b), has obvious Kelly spectral sidebands, indicating that the fiber laser is operating in the soliton regime. It has a central wavelength of 1567.3 nm and a 3 dB bandwidth of 3.1 nm. Correspondingly, as shown in Fig. 6(c), the measured auto-correlation (AC) trace can be well fitted by the hyperbolic secant function with a full width at half maximum (FWHM) of 1.46 ps, indicating that the real pulse width is about 946 fs. The time-bandwidth product is 0.358, showing that the obtained soliton pulse is almost transform limited. Figure 6(d) is the corresponding radio frequency (RF) spectrum with a resolution bandwidth (RBW) of 10 Hz. The SNR is as high as 70.7 dB, which shows stable mode-locking operation compared to the previously reported 1.5 μm pulsed laser ^[26–28]. We measured the output power of the mode-locked fiber laser, as shown in Fig. 7. It can be seen that 10% of the output power of the mode-locked fiber laser linearly increases as the pump power increases. The high SNR manifests that the MXene SA can be an ideal candidate for ultrashort pulse laser generation.

To investigate the long-term stability of the single-soliton operation, we recorded the output spectra every hour over 7 h under a fixed experimental condition, such as pump power of 60 mW, as shown in Fig. 8. Neither the central wavelength drifted nor a new wavelength component was generated during the measurement, showing excellent repeatability and long-term stability.

In summary, we have prepared the few-layer Ti3C2Tx by a simple and quick etching method. In addition, we experimentally studied the transient absorption behavior and systematically investigated the broadband saturable absorption property of Ti3C2Tx from 400 nm to 1560 nm using the Z-scan technique. We further used few-layer Ti3C2Tx as an SA to mode lock an erbium-doped fiber laser, and a stable femtosecond laser pulse has been successfully obtained with a SNR up to 70.7 dB. Our experimental results demonstrated that Ti3C2Tx could be a potential candidate for a broadband stable nonlinear optical modulator and can open new avenues toward advanced photonic devices based on MXenes.